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Re: [Nbd] [PATCH 3/4] treewide: convert PF_MEMALLOC manipulations to new helpers

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On Thu, Apr 06, 2017 at 08:38:10AM +0200, Wouter Verhelst wrote:
> On Wed, Apr 05, 2017 at 01:30:31PM +0200, Michal Hocko wrote:
> > On Wed 05-04-17 09:46:59, Vlastimil Babka wrote:
> > > We now have memalloc_noreclaim_{save,restore} helpers for robust setting and
> > > clearing of PF_MEMALLOC. Let's convert the code which was using the generic
> > > tsk_restore_flags(). No functional change.
> > 
> > It would be really great to revisit why those places outside of the mm
> > proper really need this flag. I know this is a painful exercise but I
> > wouldn't be surprised if there were abusers there.
> [...]
> > > ---
> > >  drivers/block/nbd.c      | 7 ++++---
> > >  drivers/scsi/iscsi_tcp.c | 7 ++++---
> > >  net/core/dev.c           | 7 ++++---
> > >  net/core/sock.c          | 7 ++++---
> > >  4 files changed, 16 insertions(+), 12 deletions(-)
> 
> These were all done to make swapping over network safe. The idea is that
> if a socket has SOCK_MEMALLOC set, incoming packets for that socket can
> access PFMEMALLOC reserves (whereas other sockets cannot); this all in
> the hope that one packe destined to that socket will contain the TCP ACK
> that confirms the swapout was successful and we can now release RAM
> pages for other processes.
> 
> I don't know whether they need the PF_MEMALLOC flag specifically (not a
> kernel hacker), but they do need to interact with it at any rate.
> 

At the time it was required to get access to emergency reserves so swapping
can continue. The flip side is that the memory is then protected so pages
allocated from emergency reserves are not used for network traffic that
is not involved with swap. This means that under heavy swap load, it was
perfectly possible for unrelated traffic to get dropped for quite some
time.

-- 
Mel Gorman
SUSE Labs

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[PATCH 0/4] more robust PF_MEMALLOC handling Vlastimil Babka <vbabka@suse.cz> - 2017-04-05 09:50 +0200
  [PATCH 3/4] treewide: convert PF_MEMALLOC manipulations to new helpers Vlastimil Babka <vbabka@suse.cz> - 2017-04-05 09:50 +0200
    Re: [PATCH 3/4] treewide: convert PF_MEMALLOC manipulations to new  helpers Michal Hocko <mhocko@kernel.org> - 2017-04-05 13:40 +0200
      Re: [Nbd] [PATCH 3/4] treewide: convert PF_MEMALLOC manipulations to  new helpers Wouter Verhelst <w@uter.be> - 2017-04-06 09:00 +0200
        Re: [Nbd] [PATCH 3/4] treewide: convert PF_MEMALLOC manipulations to  new helpers Mel Gorman <mgorman@techsingularity.net> - 2017-04-06 13:30 +0200
  [PATCH 1/4] mm: prevent potential recursive reclaim due to clearing PF_MEMALLOC Vlastimil Babka <vbabka@suse.cz> - 2017-04-05 09:50 +0200
    Re: [PATCH 1/4] mm: prevent potential recursive reclaim due to  clearing PF_MEMALLOC Michal Hocko <mhocko@kernel.org> - 2017-04-05 13:30 +0200
    Re: [PATCH 1/4] mm: prevent potential recursive reclaim due to clearing PF_MEMALLOC "Hillf Danton" <hillf.zj@alibaba-inc.com> - 2017-04-07 09:40 +0200
    Re: [PATCH 1/4] mm: prevent potential recursive reclaim due to  clearing PF_MEMALLOC Vlastimil Babka <vbabka@suse.cz> - 2017-04-07 11:30 +0200
  [PATCH 4/4] mtd: nand: nandsim: convert to memalloc_noreclaim_*() Vlastimil Babka <vbabka@suse.cz> - 2017-04-05 09:50 +0200
    Re: [PATCH 4/4] mtd: nand: nandsim: convert to memalloc_noreclaim_*() Vlastimil Babka <vbabka@suse.cz> - 2017-04-05 13:40 +0200
      Re: [PATCH 4/4] mtd: nand: nandsim: convert to memalloc_noreclaim_*() Michal Hocko <mhocko@kernel.org> - 2017-04-05 14:20 +0200
      Re: [PATCH 4/4] mtd: nand: nandsim: convert to memalloc_noreclaim_*() Adrian Hunter <adrian.hunter@intel.com> - 2017-04-06 08:50 +0200
        Re: [PATCH 4/4] mtd: nand: nandsim: convert to memalloc_noreclaim_*() Michal Hocko <mhocko@kernel.org> - 2017-04-06 09:30 +0200
    Re: [PATCH 4/4] mtd: nand: nandsim: convert to memalloc_noreclaim_*() Michal Hocko <mhocko@kernel.org> - 2017-04-05 13:40 +0200
      Re: [PATCH 4/4] mtd: nand: nandsim: convert to memalloc_noreclaim_*() Richard Weinberger <richard@nod.at> - 2017-04-05 13:40 +0200

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