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[PATCH v3 01/14] xfs: only allow minlen allocations when near ENOSPC

John Garry posted 14 patches 3 months ago
There is a newer version of this series
[PATCH v3 01/14] xfs: only allow minlen allocations when near ENOSPC
Posted by John Garry 3 months ago 
From: Dave Chinner <dchinner@redhat.com>

When we are near ENOSPC and don't have enough free
space for an args->maxlen allocation, xfs_alloc_space_available()
will trim args->maxlen to equal the available space. However, this
function has only checked that there is enough contiguous free space
for an aligned args->minlen allocation to succeed. Hence there is no
guarantee that an args->maxlen allocation will succeed, nor that the
available space will allow for correct alignment of an args->maxlen
allocation.

Further, by trimming args->maxlen arbitrarily, it breaks an
assumption made in xfs_alloc_fix_len() that if the caller wants
aligned allocation, then args->maxlen will be set to an aligned
value. It then skips the tail alignment and so we end up with
extents that aren't aligned to extent size hint boundaries as we
approach ENOSPC.

To avoid this problem, don't reduce args->maxlen by some random,
arbitrary amount. If args->maxlen is too large for the available
space, reduce the allocation to a minlen allocation as we know we
have contiguous free space available for this to succeed and always
be correctly aligned.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 fs/xfs/libxfs/xfs_alloc.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index 59326f84f6a5..d559d992c6ef 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -2524,14 +2524,23 @@ xfs_alloc_space_available(
 	if (available < (int)max(args->total, alloc_len))
 		return false;
 
+	if (flags & XFS_ALLOC_FLAG_CHECK)
+		return true;
+
 	/*
-	 * Clamp maxlen to the amount of free space available for the actual
-	 * extent allocation.
+	 * If we can't do a maxlen allocation, then we must reduce the size of
+	 * the allocation to match the available free space. We know how big
+	 * the largest contiguous free space we can allocate is, so that's our
+	 * upper bound. However, we don't exaclty know what alignment/size
+	 * constraints have been placed on the allocation, so we can't
+	 * arbitrarily select some new max size. Hence make this a minlen
+	 * allocation as we know that will definitely succeed and match the
+	 * callers alignment constraints.
 	 */
-	if (available < (int)args->maxlen && !(flags & XFS_ALLOC_FLAG_CHECK)) {
-		args->maxlen = available;
+	alloc_len = args->maxlen + (args->alignment - 1) + args->minalignslop;
+	if (longest < alloc_len) {
+		args->maxlen = args->minlen;
 		ASSERT(args->maxlen > 0);
-		ASSERT(args->maxlen >= args->minlen);
 	}
 
 	return true;
-- 
2.31.1
Re: [PATCH v3 01/14] xfs: only allow minlen allocations when near ENOSPC
Posted by Darrick J. Wong 2 months, 3 weeks ago 
On Thu, Aug 01, 2024 at 04:30:44PM +0000, John Garry wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> When we are near ENOSPC and don't have enough free
> space for an args->maxlen allocation, xfs_alloc_space_available()
> will trim args->maxlen to equal the available space. However, this
> function has only checked that there is enough contiguous free space
> for an aligned args->minlen allocation to succeed. Hence there is no
> guarantee that an args->maxlen allocation will succeed, nor that the
> available space will allow for correct alignment of an args->maxlen
> allocation.
> 
> Further, by trimming args->maxlen arbitrarily, it breaks an
> assumption made in xfs_alloc_fix_len() that if the caller wants
> aligned allocation, then args->maxlen will be set to an aligned
> value. It then skips the tail alignment and so we end up with
> extents that aren't aligned to extent size hint boundaries as we
> approach ENOSPC.
> 
> To avoid this problem, don't reduce args->maxlen by some random,
> arbitrary amount. If args->maxlen is too large for the available
> space, reduce the allocation to a minlen allocation as we know we
> have contiguous free space available for this to succeed and always
> be correctly aligned.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_alloc.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 59326f84f6a5..d559d992c6ef 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -2524,14 +2524,23 @@ xfs_alloc_space_available(
>  	if (available < (int)max(args->total, alloc_len))
>  		return false;
>  
> +	if (flags & XFS_ALLOC_FLAG_CHECK)
> +		return true;
> +
>  	/*
> -	 * Clamp maxlen to the amount of free space available for the actual
> -	 * extent allocation.
> +	 * If we can't do a maxlen allocation, then we must reduce the size of
> +	 * the allocation to match the available free space. We know how big
> +	 * the largest contiguous free space we can allocate is, so that's our
> +	 * upper bound. However, we don't exaclty know what alignment/size
> +	 * constraints have been placed on the allocation, so we can't
> +	 * arbitrarily select some new max size. Hence make this a minlen
> +	 * allocation as we know that will definitely succeed and match the
> +	 * callers alignment constraints.
>  	 */
> -	if (available < (int)args->maxlen && !(flags & XFS_ALLOC_FLAG_CHECK)) {
> -		args->maxlen = available;
> +	alloc_len = args->maxlen + (args->alignment - 1) + args->minalignslop;
> +	if (longest < alloc_len) {
> +		args->maxlen = args->minlen;

Same question as the June 21st posting:

Is it possible to reduce maxlen the largest multiple of the alignment
that is still less than @longest?

--D

>  		ASSERT(args->maxlen > 0);
> -		ASSERT(args->maxlen >= args->minlen);
>  	}
>  
>  	return true;
> -- 
> 2.31.1
> 
>
Re: [PATCH v3 01/14] xfs: only allow minlen allocations when near ENOSPC
Posted by Dave Chinner 2 months, 3 weeks ago 
On Tue, Aug 06, 2024 at 11:51:38AM -0700, Darrick J. Wong wrote:
> On Thu, Aug 01, 2024 at 04:30:44PM +0000, John Garry wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > When we are near ENOSPC and don't have enough free
> > space for an args->maxlen allocation, xfs_alloc_space_available()
> > will trim args->maxlen to equal the available space. However, this
> > function has only checked that there is enough contiguous free space
> > for an aligned args->minlen allocation to succeed. Hence there is no
> > guarantee that an args->maxlen allocation will succeed, nor that the
> > available space will allow for correct alignment of an args->maxlen
> > allocation.
> > 
> > Further, by trimming args->maxlen arbitrarily, it breaks an
> > assumption made in xfs_alloc_fix_len() that if the caller wants
> > aligned allocation, then args->maxlen will be set to an aligned
> > value. It then skips the tail alignment and so we end up with
> > extents that aren't aligned to extent size hint boundaries as we
> > approach ENOSPC.
> > 
> > To avoid this problem, don't reduce args->maxlen by some random,
> > arbitrary amount. If args->maxlen is too large for the available
> > space, reduce the allocation to a minlen allocation as we know we
> > have contiguous free space available for this to succeed and always
> > be correctly aligned.
> > 
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > Signed-off-by: John Garry <john.g.garry@oracle.com>
> > ---
> >  fs/xfs/libxfs/xfs_alloc.c | 19 ++++++++++++++-----
> >  1 file changed, 14 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > index 59326f84f6a5..d559d992c6ef 100644
> > --- a/fs/xfs/libxfs/xfs_alloc.c
> > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > @@ -2524,14 +2524,23 @@ xfs_alloc_space_available(
> >  	if (available < (int)max(args->total, alloc_len))
> >  		return false;
> >  
> > +	if (flags & XFS_ALLOC_FLAG_CHECK)
> > +		return true;
> > +
> >  	/*
> > -	 * Clamp maxlen to the amount of free space available for the actual
> > -	 * extent allocation.
> > +	 * If we can't do a maxlen allocation, then we must reduce the size of
> > +	 * the allocation to match the available free space. We know how big
> > +	 * the largest contiguous free space we can allocate is, so that's our
> > +	 * upper bound. However, we don't exaclty know what alignment/size
> > +	 * constraints have been placed on the allocation, so we can't
> > +	 * arbitrarily select some new max size. Hence make this a minlen
> > +	 * allocation as we know that will definitely succeed and match the
> > +	 * callers alignment constraints.
> >  	 */
> > -	if (available < (int)args->maxlen && !(flags & XFS_ALLOC_FLAG_CHECK)) {
> > -		args->maxlen = available;
> > +	alloc_len = args->maxlen + (args->alignment - 1) + args->minalignslop;
> > +	if (longest < alloc_len) {
> > +		args->maxlen = args->minlen;
> 
> Same question as the June 21st posting:
> 
> Is it possible to reduce maxlen the largest multiple of the alignment
> that is still less than @longest?

Perhaps.

The comment does say "we don't exaclty know what alignment/size
constraints have been placed on the allocation, so we can't
arbitrarily select some new max size."

Given this unknown I simply punted the issue and went straight to
selecting a size the caller has guaranteed will be valid for their
constraints.

-Dave.

-- 
Dave Chinner
david@fromorbit.com

Patchew 2.1-devel-b67e4c2

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