Groups > linux.kernel > #1603848 > unrolled thread

[patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop

Started by	Kirill Smelkov <kirr@nexedi.com>
First post	2017-03-18 21:00 +0100
Last post	2017-03-20 21:10 +0100
Articles	4 — 2 participants

Back to article view | Back to linux.kernel

  [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop Kirill Smelkov <kirr@nexedi.com> - 2017-03-18 21:00 +0100
    Why mmap(MAP_POPULATE | MAP_NONBLOCK) is needed  (Re: [patch]  mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be  noop) Kirill Smelkov <kirr@nexedi.com> - 2017-03-20 17:10 +0100
      Re: Why mmap(MAP_POPULATE | MAP_NONBLOCK) is needed (Re: [patch]  mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop) "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2017-03-20 20:50 +0100
        Re: Why mmap(MAP_POPULATE | MAP_NONBLOCK) is needed (Re: [patch]  mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be  noop) Kirill Smelkov <kirr@nexedi.com> - 2017-03-20 21:10 +0100

#1603848 — [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop

From	Kirill Smelkov <kirr@nexedi.com>
Date	2017-03-18 21:00 +0100
Subject	[patch] mmap.2: Add link to commit which broke MAP_POPULATE \| MAP_NONBLOCK to be noop
Message-ID	<tmsG5-2No-7@gated-at.bofh.it>

Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
---
 man2/mmap.2 | 1 +
 1 file changed, 1 insertion(+)

diff --git a/man2/mmap.2 b/man2/mmap.2
index 96875e486..f6fd56523 100644
--- a/man2/mmap.2
+++ b/man2/mmap.2
@@ -300,6 +300,7 @@ Don't perform read-ahead:
 create page tables entries only for pages
 that are already present in RAM.
 Since Linux 2.6.23, this flag causes
+.\" commit 54cb8821de07f2ffcd28c380ce9b93d5784b40d7
 .BR MAP_POPULATE
 to do nothing.
 One day, the combination of
-- 
2.11.0

[toc] | [next] | [standalone]

#1604726 — Why mmap(MAP_POPULATE | MAP_NONBLOCK) is needed (Re: [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop)

From	Kirill Smelkov <kirr@nexedi.com>
Date	2017-03-20 17:10 +0100
Subject	Why mmap(MAP_POPULATE \| MAP_NONBLOCK) is needed (Re: [patch] mmap.2: Add link to commit which broke MAP_POPULATE \| MAP_NONBLOCK to be noop)
Message-ID	<tn82C-77f-23@gated-at.bofh.it>
In reply to	#1603848

On Sat, Mar 18, 2017 at 10:40:10PM +0300, Kirill Smelkov wrote:
> Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
> ---
>  man2/mmap.2 | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/man2/mmap.2 b/man2/mmap.2
> index 96875e486..f6fd56523 100644
> --- a/man2/mmap.2
> +++ b/man2/mmap.2
> @@ -300,6 +300,7 @@ Don't perform read-ahead:
>  create page tables entries only for pages
>  that are already present in RAM.
>  Since Linux 2.6.23, this flag causes
> +.\" commit 54cb8821de07f2ffcd28c380ce9b93d5784b40d7
>  .BR MAP_POPULATE
>  to do nothing.
>  One day, the combination of

Please also find below benchmark which explains why

	mmap(MAP_POPULATE | MAP_NONBLOCK)
	
is actually needed.

Thanks,
Kirill

---- 8< ---- (https://lab.nexedi.com/kirr/misc/blob/5a25f4ae/t_sysmmap_c.c)
/* This program benchmarks pagefault time.
 *
 * Unfortunately as of 2017-Mar-20 for data in pagecache the situation is as
 * follows (i7-6600U, Linux 4.9.13):
 *
 * 1. minor pagefault:			~ 1200ns
 *    (this program)
 *
 * 2. read syscall + whole page copy:	~  215ns
 *    (https://github.com/golang/go/issues/19563#issuecomment-287423654)
 *
 * 3. it is not possible to mmap(MAP_POPULATE | MAP_NONBLOCK) (i.e. prefault
 *    those PTE that are already in pagecache).
 *    ( http://www.spinics.net/lists/linux-man/msg11420.html,
 *      https://git.kernel.org/linus/54cb8821de07f2ffcd28c380ce9b93d5784b40d7 )
 *
 * 4. (Q) I'm not sure a mechanism exists in the kernel to automatically
 *    subscribe a VMA so that when a page becomes pagecached, associated PTE is
 *    adjusted so that programs won't need to pay minor pagefault time on
 *    access.
 *
 * unless 3 and 4 are solved mmap unfortunately seems to be slower choice
 * compared to just pread.
 */
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/user.h>
#include <sys/mman.h>

//		 12345678
#define	NITER	500000

// microtime returns current time as double
double microtime() {
	int err;
	struct timeval tv;

	err = gettimeofday(&tv, NULL);
	if (err == -1) {
		perror("gettimeofday");
		abort();
	}

	return tv.tv_sec + 1E-6 * tv.tv_usec;
}


int main() {
	unsigned char *addr, sum = 0;
	int fd, err, i;
	size_t size;
	double Tstart, Tend;

	fd = open("/dev/shm/y.dat", O_RDWR | O_CREAT | O_TRUNC, 0666);
	if (fd == -1) {
		perror("open");
		abort();
	}

	size = NITER * PAGE_SIZE;

	err = ftruncate(fd, size);
	if (err == -1) {
		perror("ftruncate");
		abort();
	}

#if 1
	// make sure RAM is actually allocated
	Tstart = microtime();
	err = fallocate(fd, /*mode*/0, 0, size);
	Tend = microtime();
	if (err == -1) {
		perror("fallocate");
		abort();
	}
	printf("T(fallocate):\t%.1f\t%6.1f ns / page\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER);
#endif

	Tstart = microtime();
	addr = mmap(NULL, size, PROT_READ, MAP_SHARED, fd, 0);
	//addr = mmap(NULL, size, PROT_READ, MAP_SHARED | MAP_POPULATE, fd, 0);
	//addr = mmap(NULL, size, PROT_READ, MAP_SHARED | MAP_POPULATE | MAP_NONBLOCK, fd, 0);
	if (addr == MAP_FAILED) {
		perror("mmap");
		abort();
	}
	Tend = microtime();
	printf("T(mmap):\t%.1f\t%6.1f ns / page\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER);

	Tstart = microtime();
	//for (int j=0; j < 100; j++)
	for (i=0; i<NITER; i++) {
		sum += addr[i*PAGE_SIZE];
	}
	Tend = microtime();

	printf("T(pagefault):\t%.1f\t%6.1f ns / page\t(%i)\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER, sum);

	return 0;
}
---- 8< ----

[toc] | [prev] | [next] | [standalone]

#1605095 — Re: Why mmap(MAP_POPULATE | MAP_NONBLOCK) is needed (Re: [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop)

From	"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Date	2017-03-20 20:50 +0100
Subject	Re: Why mmap(MAP_POPULATE \| MAP_NONBLOCK) is needed (Re: [patch] mmap.2: Add link to commit which broke MAP_POPULATE \| MAP_NONBLOCK to be noop)
Message-ID	<tnbtw-Zn-15@gated-at.bofh.it>
In reply to	#1604726

[CC += Michel Lespinasse <walken@google.com>]

Kirill,

I need some help here.

On 20 March 2017 at 16:59, Kirill Smelkov <kirr@nexedi.com> wrote:
> On Sat, Mar 18, 2017 at 10:40:10PM +0300, Kirill Smelkov wrote:
>> Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
>> ---
>>  man2/mmap.2 | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/man2/mmap.2 b/man2/mmap.2
>> index 96875e486..f6fd56523 100644
>> --- a/man2/mmap.2
>> +++ b/man2/mmap.2
>> @@ -300,6 +300,7 @@ Don't perform read-ahead:
>>  create page tables entries only for pages
>>  that are already present in RAM.
>>  Since Linux 2.6.23, this flag causes
>> +.\" commit 54cb8821de07f2ffcd28c380ce9b93d5784b40d7
>>  .BR MAP_POPULATE
>>  to do nothing.
>>  One day, the combination of
>
> Please also find below benchmark which explains why
>
>         mmap(MAP_POPULATE | MAP_NONBLOCK)
>
> is actually needed.

Okay -- clearly things have changed (but I received no man-pages
patch). What do you believe the man page should now say.

Or, perhaps we can ask Michel:

commit bebeb3d68b24bb4132d452c5707fe321208bcbcd
Author: Michel Lespinasse <walken@google.com>
Date:   Fri Feb 22 16:32:37 2013 -0800

The above commit (which went into Linux 3.9) seems to be the source of
the change.

Michael, can you suggest to us what the mmap() man page should now say
about MAP_POPULATE?

Thanks,

Michael


>
> Thanks,
> Kirill
>
> ---- 8< ---- (https://lab.nexedi.com/kirr/misc/blob/5a25f4ae/t_sysmmap_c.c)
> /* This program benchmarks pagefault time.
>  *
>  * Unfortunately as of 2017-Mar-20 for data in pagecache the situation is as
>  * follows (i7-6600U, Linux 4.9.13):
>  *
>  * 1. minor pagefault:                  ~ 1200ns
>  *    (this program)
>  *
>  * 2. read syscall + whole page copy:   ~  215ns
>  *    (https://github.com/golang/go/issues/19563#issuecomment-287423654)
>  *
>  * 3. it is not possible to mmap(MAP_POPULATE | MAP_NONBLOCK) (i.e. prefault
>  *    those PTE that are already in pagecache).
>  *    ( http://www.spinics.net/lists/linux-man/msg11420.html,
>  *      https://git.kernel.org/linus/54cb8821de07f2ffcd28c380ce9b93d5784b40d7 )
>  *
>  * 4. (Q) I'm not sure a mechanism exists in the kernel to automatically
>  *    subscribe a VMA so that when a page becomes pagecached, associated PTE is
>  *    adjusted so that programs won't need to pay minor pagefault time on
>  *    access.
>  *
>  * unless 3 and 4 are solved mmap unfortunately seems to be slower choice
>  * compared to just pread.
>  */
> #define _GNU_SOURCE
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/time.h>
> #include <sys/user.h>
> #include <sys/mman.h>
>
> //               12345678
> #define NITER   500000
>
> // microtime returns current time as double
> double microtime() {
>         int err;
>         struct timeval tv;
>
>         err = gettimeofday(&tv, NULL);
>         if (err == -1) {
>                 perror("gettimeofday");
>                 abort();
>         }
>
>         return tv.tv_sec + 1E-6 * tv.tv_usec;
> }
>
>
> int main() {
>         unsigned char *addr, sum = 0;
>         int fd, err, i;
>         size_t size;
>         double Tstart, Tend;
>
>         fd = open("/dev/shm/y.dat", O_RDWR | O_CREAT | O_TRUNC, 0666);
>         if (fd == -1) {
>                 perror("open");
>                 abort();
>         }
>
>         size = NITER * PAGE_SIZE;
>
>         err = ftruncate(fd, size);
>         if (err == -1) {
>                 perror("ftruncate");
>                 abort();
>         }
>
> #if 1
>         // make sure RAM is actually allocated
>         Tstart = microtime();
>         err = fallocate(fd, /*mode*/0, 0, size);
>         Tend = microtime();
>         if (err == -1) {
>                 perror("fallocate");
>                 abort();
>         }
>         printf("T(fallocate):\t%.1f\t%6.1f ns / page\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER);
> #endif
>
>         Tstart = microtime();
>         addr = mmap(NULL, size, PROT_READ, MAP_SHARED, fd, 0);
>         //addr = mmap(NULL, size, PROT_READ, MAP_SHARED | MAP_POPULATE, fd, 0);
>         //addr = mmap(NULL, size, PROT_READ, MAP_SHARED | MAP_POPULATE | MAP_NONBLOCK, fd, 0);
>         if (addr == MAP_FAILED) {
>                 perror("mmap");
>                 abort();
>         }
>         Tend = microtime();
>         printf("T(mmap):\t%.1f\t%6.1f ns / page\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER);
>
>         Tstart = microtime();
>         //for (int j=0; j < 100; j++)
>         for (i=0; i<NITER; i++) {
>                 sum += addr[i*PAGE_SIZE];
>         }
>         Tend = microtime();
>
>         printf("T(pagefault):\t%.1f\t%6.1f ns / page\t(%i)\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER, sum);
>
>         return 0;
> }
> ---- 8< ----



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

[toc] | [prev] | [next] | [standalone]

#1605104 — Re: Why mmap(MAP_POPULATE | MAP_NONBLOCK) is needed (Re: [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop)

From	Kirill Smelkov <kirr@nexedi.com>
Date	2017-03-20 21:10 +0100
Subject	Re: Why mmap(MAP_POPULATE \| MAP_NONBLOCK) is needed (Re: [patch] mmap.2: Add link to commit which broke MAP_POPULATE \| MAP_NONBLOCK to be noop)
Message-ID	<tnbMS-1ns-11@gated-at.bofh.it>
In reply to	#1605095

Michael, first of all thanks for feedback.

On Mon, Mar 20, 2017 at 08:38:50PM +0100, Michael Kerrisk (man-pages) wrote:
> [CC += Michel Lespinasse <walken@google.com>]
> 
> Kirill,
> 
> I need some help here.
> 
> On 20 March 2017 at 16:59, Kirill Smelkov <kirr@nexedi.com> wrote:
> > On Sat, Mar 18, 2017 at 10:40:10PM +0300, Kirill Smelkov wrote:
> >> Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
> >> ---
> >>  man2/mmap.2 | 1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >> diff --git a/man2/mmap.2 b/man2/mmap.2
> >> index 96875e486..f6fd56523 100644
> >> --- a/man2/mmap.2
> >> +++ b/man2/mmap.2
> >> @@ -300,6 +300,7 @@ Don't perform read-ahead:
> >>  create page tables entries only for pages
> >>  that are already present in RAM.
> >>  Since Linux 2.6.23, this flag causes
> >> +.\" commit 54cb8821de07f2ffcd28c380ce9b93d5784b40d7
> >>  .BR MAP_POPULATE
> >>  to do nothing.
> >>  One day, the combination of
> >
> > Please also find below benchmark which explains why
> >
> >         mmap(MAP_POPULATE | MAP_NONBLOCK)
> >
> > is actually needed.
> 
> Okay -- clearly things have changed (but I received no man-pages
> patch).

Strange it was sent. Let me show it once again here (git am -s):

---- 8< ----
From: Kirill Smelkov <kirr@nexedi.com>
Subject: [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop

Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
---
 man2/mmap.2 | 1 +
 1 file changed, 1 insertion(+)

diff --git a/man2/mmap.2 b/man2/mmap.2
index 96875e486..f6fd56523 100644
--- a/man2/mmap.2
+++ b/man2/mmap.2
@@ -300,6 +300,7 @@ Don't perform read-ahead:
 create page tables entries only for pages
 that are already present in RAM.
 Since Linux 2.6.23, this flag causes
+.\" commit 54cb8821de07f2ffcd28c380ce9b93d5784b40d7
 .BR MAP_POPULATE
 to do nothing.
 One day, the combination of
--
2.11.0
---- 8< ----


> What do you believe the man page should now say.

What man page says today correctly describes current behaviour:

---- 8< ----
MAP_NONBLOCK (since Linux 2.5.46)
       This flag is meaningful only in conjunction  with  MAP_POPULATE.   Don't  perform  read-
       ahead: create page tables entries only for pages that are already present in RAM.  Since
       Linux 2.6.23, this flag causes MAP_POPULATE to do nothing.  One day, the combination  of
       MAP_POPULATE and MAP_NONBLOCK may be reimplemented.
---- 8< ----

For now I've just added reference to commit corresponding to "Since Linux
2.6.23, this flag causes MAP_POPULATE to do nothing."


> Or, perhaps we can ask Michel:
> 
> commit bebeb3d68b24bb4132d452c5707fe321208bcbcd
> Author: Michel Lespinasse <walken@google.com>
> Date:   Fri Feb 22 16:32:37 2013 -0800
> 
> The above commit (which went into Linux 3.9) seems to be the source of
> the change.
> 
> Michael, can you suggest to us what the mmap() man page should now say
> about MAP_POPULATE?

It is good to have feedback from relevant people, but as my patch to
man-pages says, if I understand it correctly, the original patch which
changed behaviour is this:

---- 8< ----
commit 54cb8821de07f2ffcd28c380ce9b93d5784b40d7
Author: Nick Piggin <npiggin@suse.de>
Date:   Thu Jul 19 01:46:59 2007 -0700

    mm: merge populate and nopage into fault (fixes nonlinear)

    ...

    After this patch, MAP_NONBLOCK no longer sets up ptes for pages present in		<-- NOTE here
    pagecache.  Seems like a fringe functionality anyway.

    ...

    [akpm@linux-foundation.org: cleanup]
    [randy.dunlap@oracle.com: doc. fixes for readahead]
    [akpm@linux-foundation.org: build fix]
    Signed-off-by: Nick Piggin <npiggin@suse.de>
    Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
    Cc: Mark Fasheh <mark.fasheh@oracle.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---- 8< ----

Adding all people involved to Cc - please have a look at quoted benchmark below
which justifies usage of mmap(MAP_POPULATE | MAP_NONBLOCK).

Thanks,
Kirill


> > ---- 8< ---- (https://lab.nexedi.com/kirr/misc/blob/5a25f4ae/t_sysmmap_c.c)
> > /* This program benchmarks pagefault time.
> >  *
> >  * Unfortunately as of 2017-Mar-20 for data in pagecache the situation is as
> >  * follows (i7-6600U, Linux 4.9.13):
> >  *
> >  * 1. minor pagefault:                  ~ 1200ns
> >  *    (this program)
> >  *
> >  * 2. read syscall + whole page copy:   ~  215ns
> >  *    (https://github.com/golang/go/issues/19563#issuecomment-287423654)
> >  *
> >  * 3. it is not possible to mmap(MAP_POPULATE | MAP_NONBLOCK) (i.e. prefault
> >  *    those PTE that are already in pagecache).
> >  *    ( http://www.spinics.net/lists/linux-man/msg11420.html,
> >  *      https://git.kernel.org/linus/54cb8821de07f2ffcd28c380ce9b93d5784b40d7 )
> >  *
> >  * 4. (Q) I'm not sure a mechanism exists in the kernel to automatically
> >  *    subscribe a VMA so that when a page becomes pagecached, associated PTE is
> >  *    adjusted so that programs won't need to pay minor pagefault time on
> >  *    access.
> >  *
> >  * unless 3 and 4 are solved mmap unfortunately seems to be slower choice
> >  * compared to just pread.
> >  */
> > #define _GNU_SOURCE
> > #include <sys/types.h>
> > #include <sys/stat.h>
> > #include <fcntl.h>
> > #include <unistd.h>
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <sys/time.h>
> > #include <sys/user.h>
> > #include <sys/mman.h>
> >
> > //               12345678
> > #define NITER   500000
> >
> > // microtime returns current time as double
> > double microtime() {
> >         int err;
> >         struct timeval tv;
> >
> >         err = gettimeofday(&tv, NULL);
> >         if (err == -1) {
> >                 perror("gettimeofday");
> >                 abort();
> >         }
> >
> >         return tv.tv_sec + 1E-6 * tv.tv_usec;
> > }
> >
> >
> > int main() {
> >         unsigned char *addr, sum = 0;
> >         int fd, err, i;
> >         size_t size;
> >         double Tstart, Tend;
> >
> >         fd = open("/dev/shm/y.dat", O_RDWR | O_CREAT | O_TRUNC, 0666);
> >         if (fd == -1) {
> >                 perror("open");
> >                 abort();
> >         }
> >
> >         size = NITER * PAGE_SIZE;
> >
> >         err = ftruncate(fd, size);
> >         if (err == -1) {
> >                 perror("ftruncate");
> >                 abort();
> >         }
> >
> > #if 1
> >         // make sure RAM is actually allocated
> >         Tstart = microtime();
> >         err = fallocate(fd, /*mode*/0, 0, size);
> >         Tend = microtime();
> >         if (err == -1) {
> >                 perror("fallocate");
> >                 abort();
> >         }
> >         printf("T(fallocate):\t%.1f\t%6.1f ns / page\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER);
> > #endif
> >
> >         Tstart = microtime();
> >         addr = mmap(NULL, size, PROT_READ, MAP_SHARED, fd, 0);
> >         //addr = mmap(NULL, size, PROT_READ, MAP_SHARED | MAP_POPULATE, fd, 0);
> >         //addr = mmap(NULL, size, PROT_READ, MAP_SHARED | MAP_POPULATE | MAP_NONBLOCK, fd, 0);
> >         if (addr == MAP_FAILED) {
> >                 perror("mmap");
> >                 abort();
> >         }
> >         Tend = microtime();
> >         printf("T(mmap):\t%.1f\t%6.1f ns / page\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER);
> >
> >         Tstart = microtime();
> >         //for (int j=0; j < 100; j++)
> >         for (i=0; i<NITER; i++) {
> >                 sum += addr[i*PAGE_SIZE];
> >         }
> >         Tend = microtime();
> >
> >         printf("T(pagefault):\t%.1f\t%6.1f ns / page\t(%i)\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER, sum);
> >
> >         return 0;
> > }
> > ---- 8< ----

[toc] | [prev] | [standalone]

csiph-web

[patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop

Contents

#1603848 — [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop

#1604726 — Why mmap(MAP_POPULATE | MAP_NONBLOCK) is needed (Re: [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop)

#1605095 — Re: Why mmap(MAP_POPULATE | MAP_NONBLOCK) is needed (Re: [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop)

#1605104 — Re: Why mmap(MAP_POPULATE | MAP_NONBLOCK) is needed (Re: [patch] mmap.2: Add link to commit which broke MAP_POPULATE | MAP_NONBLOCK to be noop)