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Re: [PATCH] tmpfs: truncate at i_size

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  Re: [PATCH] tmpfs: truncate at i_size Hugh Dickins <hughd@google.com> - 2015-06-16 22:10 +0200
    [PATCH] tmpfs: truncate prealloc blocks past i_size Hugh Dickins <hughd@google.com> - 2015-06-16 22:20 +0200

#1166357 — Re: [PATCH] tmpfs: truncate at i_size

On Tue, 19 May 2015, Josef Bacik wrote:

> If we fallocate past i_size with KEEP_SIZE, extend the file to use some but not
> all of this space, and then truncate(i_size) we won't trim the excess
> preallocated space.  We decided at LSF that we want to truncate the fallocated
> bit past i_size when we truncate to i_size, which is what this patch does.
> Thanks,
> 
> Signed-off-by: Josef Bacik <jbacik@fb.com>

Sorry for the delay, it's been on my mind but only now I get to it.
Yes, that was agreed at LSF, and I've checked that indeed tmpfs is
out of line here: thank you for fixing it.  But I do prefer your
original more explicit description, so I'll send the patch to akpm
now for v4.2, with that description instead (plus a reference to LSF).

Thanks,
Hugh

> ---
>  mm/shmem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index de98137..089afde 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -569,7 +569,7 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
>  			i_size_write(inode, newsize);
>  			inode->i_ctime = inode->i_mtime = CURRENT_TIME;
>  		}
> -		if (newsize < oldsize) {
> +		if (newsize <= oldsize) {
>  			loff_t holebegin = round_up(newsize, PAGE_SIZE);
>  			unmap_mapping_range(inode->i_mapping, holebegin, 0, 1);
>  			shmem_truncate_range(inode, newsize, (loff_t)-1);
> -- 
> 1.8.3.1
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#1166366 — [PATCH] tmpfs: truncate prealloc blocks past i_size

From: Josef Bacik <jbacik@fb.com>

One of the rocksdb people noticed that when you do something like this

fallocate(fd, FALLOC_FL_KEEP_SIZE, 0, 10M)
pwrite(fd, buf, 5M, 0)
ftruncate(5M)

on tmpfs, the file would still take up 10M: which led to super fun issues
because we were getting ENOSPC before we thought we should be getting
ENOSPC.  This patch fixes the problem, and mirrors what all the other
fs'es do (and was agreed to be the correct behaviour at LSF).

I tested it locally to make sure it worked properly with the following

xfs_io -f -c "falloc -k 0 10M" -c "pwrite 0 5M" -c "truncate 5M" file

Without the patch we have "Blocks: 20480", with the patch we have the
correct value of "Blocks: 10240".

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -569,7 +569,7 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
 			i_size_write(inode, newsize);
 			inode->i_ctime = inode->i_mtime = CURRENT_TIME;
 		}
-		if (newsize < oldsize) {
+		if (newsize <= oldsize) {
 			loff_t holebegin = round_up(newsize, PAGE_SIZE);
 			unmap_mapping_range(inode->i_mapping, holebegin, 0, 1);
 			shmem_truncate_range(inode, newsize, (loff_t)-1);
-- 
1.8.3.1
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