[PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Vratislav Bendel]

The function xfs_buftarg_isolate() used by xfs buffer schrinkers 
to determine whether a buffer should be isolated and disposed 
from LRU list, has inverted logic.

Excerpt from xfs_buftarg_isolate():
        /*
         * Decrement the b_lru_ref count unless the value is already
         * zero. If the value is already zero, we need to reclaim the
         * buffer, otherwise it gets another trip through the LRU.
         */
        if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
                spin_unlock(&bp->b_lock);
                return LRU_ROTATE;
        }

However, as per documentation, atomic_add_unless() returns _zero_
if the atomic value was originally equal to the specified *unsless* value.

Ultimately causing a xfs_buffer with ->b_lru_ref == 0, to take another 
trip around LRU, while isolating buffers with non-zero b_lru_ref.

Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
CC: Brian Foster <bfoster@redhat.com>
---
 fs/xfs/xfs_buf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index d1da2ee9e6db..ac669a10c62f 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -1708,7 +1708,7 @@ xfs_buftarg_isolate(
 	 * zero. If the value is already zero, we need to reclaim the
 	 * buffer, otherwise it gets another trip through the LRU.
 	 */
-	if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
+	if (atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
 		spin_unlock(&bp->b_lock);
 		return LRU_ROTATE;
 	}
-- 
2.14.3

Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Luis R. Rodriguez]

On Wed, Feb 28, 2018 at 04:49:51PM +0100, Vratislav Bendel wrote:
> The function xfs_buftarg_isolate() used by xfs buffer schrinkers 
> to determine whether a buffer should be isolated and disposed 
> from LRU list, has inverted logic.
> 
> Excerpt from xfs_buftarg_isolate():
>         /*
>          * Decrement the b_lru_ref count unless the value is already
>          * zero. If the value is already zero, we need to reclaim the
>          * buffer, otherwise it gets another trip through the LRU.
>          */
>         if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
>                 spin_unlock(&bp->b_lock);
>                 return LRU_ROTATE;
>         }
> 
> However, as per documentation, atomic_add_unless() returns _zero_
> if the atomic value was originally equal to the specified *unsless* value.
> 
> Ultimately causing a xfs_buffer with ->b_lru_ref == 0, to take another 
> trip around LRU, while isolating buffers with non-zero b_lru_ref.
> 
> Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
> CC: Brian Foster <bfoster@redhat.com>

Can you add a respective Fixes: tag?  Also what effects are observed by
the user when this happens on the kernel log?

 Luis

Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Brian Foster]

On Wed, Feb 28, 2018 at 04:49:51PM +0100, Vratislav Bendel wrote:
> The function xfs_buftarg_isolate() used by xfs buffer schrinkers 
> to determine whether a buffer should be isolated and disposed 
> from LRU list, has inverted logic.
> 
> Excerpt from xfs_buftarg_isolate():
>         /*
>          * Decrement the b_lru_ref count unless the value is already
>          * zero. If the value is already zero, we need to reclaim the
>          * buffer, otherwise it gets another trip through the LRU.
>          */
>         if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
>                 spin_unlock(&bp->b_lock);
>                 return LRU_ROTATE;
>         }
> 
> However, as per documentation, atomic_add_unless() returns _zero_
> if the atomic value was originally equal to the specified *unsless* value.
> 

Nit:							     unless

> Ultimately causing a xfs_buffer with ->b_lru_ref == 0, to take another 
> trip around LRU, while isolating buffers with non-zero b_lru_ref.
> 
> Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
> CC: Brian Foster <bfoster@redhat.com>
> ---

It might be worth pointing out in the commit log that currently isolated
buffers end up right back on the LRU once they are released, because
->b_lru_ref remains elevated. Therefore, this patch essentially fixes
that circuitous route by leaving them on the LRU as originally intended.
Otherwise this looks Ok to me:

Reviewed-by: Brian Foster <bfoster@redhat.com>

Thanks for sending the patch.

Brian

>  fs/xfs/xfs_buf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index d1da2ee9e6db..ac669a10c62f 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -1708,7 +1708,7 @@ xfs_buftarg_isolate(
>  	 * zero. If the value is already zero, we need to reclaim the
>  	 * buffer, otherwise it gets another trip through the LRU.
>  	 */
> -	if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
> +	if (atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
>  		spin_unlock(&bp->b_lock);
>  		return LRU_ROTATE;
>  	}
> -- 
> 2.14.3
> 
> --
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Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Darrick J. Wong]

On Wed, Feb 28, 2018 at 04:49:51PM +0100, Vratislav Bendel wrote:
> The function xfs_buftarg_isolate() used by xfs buffer schrinkers 
> to determine whether a buffer should be isolated and disposed 
> from LRU list, has inverted logic.
> 
> Excerpt from xfs_buftarg_isolate():
>         /*
>          * Decrement the b_lru_ref count unless the value is already
>          * zero. If the value is already zero, we need to reclaim the
>          * buffer, otherwise it gets another trip through the LRU.
>          */
>         if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
>                 spin_unlock(&bp->b_lock);
>                 return LRU_ROTATE;
>         }
> 
> However, as per documentation, atomic_add_unless() returns _zero_
> if the atomic value was originally equal to the specified *unsless* value.
> 
> Ultimately causing a xfs_buffer with ->b_lru_ref == 0, to take another 
> trip around LRU, while isolating buffers with non-zero b_lru_ref.
> 
> Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
> CC: Brian Foster <bfoster@redhat.com>

Looks ok, will test...
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/xfs/xfs_buf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index d1da2ee9e6db..ac669a10c62f 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -1708,7 +1708,7 @@ xfs_buftarg_isolate(
>  	 * zero. If the value is already zero, we need to reclaim the
>  	 * buffer, otherwise it gets another trip through the LRU.
>  	 */
> -	if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
> +	if (atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
>  		spin_unlock(&bp->b_lock);
>  		return LRU_ROTATE;
>  	}
> -- 
> 2.14.3
> 

Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Darrick J. Wong]

On Thu, Mar 01, 2018 at 02:48:00PM -0800, Darrick J. Wong wrote:
> On Wed, Feb 28, 2018 at 04:49:51PM +0100, Vratislav Bendel wrote:
> > The function xfs_buftarg_isolate() used by xfs buffer schrinkers 
> > to determine whether a buffer should be isolated and disposed 
> > from LRU list, has inverted logic.
> > 
> > Excerpt from xfs_buftarg_isolate():
> >         /*
> >          * Decrement the b_lru_ref count unless the value is already
> >          * zero. If the value is already zero, we need to reclaim the
> >          * buffer, otherwise it gets another trip through the LRU.
> >          */
> >         if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
> >                 spin_unlock(&bp->b_lock);
> >                 return LRU_ROTATE;
> >         }
> > 
> > However, as per documentation, atomic_add_unless() returns _zero_
> > if the atomic value was originally equal to the specified *unsless* value.
> > 
> > Ultimately causing a xfs_buffer with ->b_lru_ref == 0, to take another 
> > trip around LRU, while isolating buffers with non-zero b_lru_ref.
> > 
> > Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
> > CC: Brian Foster <bfoster@redhat.com>
> 
> Looks ok, will test...
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

This tests ok, but please address Brian and Luis' comments before I put
this in the upstream tream.

--D

> --D
> 
> > ---
> >  fs/xfs/xfs_buf.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > index d1da2ee9e6db..ac669a10c62f 100644
> > --- a/fs/xfs/xfs_buf.c
> > +++ b/fs/xfs/xfs_buf.c
> > @@ -1708,7 +1708,7 @@ xfs_buftarg_isolate(
> >  	 * zero. If the value is already zero, we need to reclaim the
> >  	 * buffer, otherwise it gets another trip through the LRU.
> >  	 */
> > -	if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
> > +	if (atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
> >  		spin_unlock(&bp->b_lock);
> >  		return LRU_ROTATE;
> >  	}
> > -- 
> > 2.14.3
> > 
> --
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Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Vratislav Bendel]

(In response to Luis' comment:)
> Can you add a respective Fixes: tag?

It was apparently present since LRU was added to xfs buffer cache via:
commit 430cbeb86fdcbbdabea7d4aa65307de8de425350
[xfs: add a lru to the XFS buffer cache]

But I wouldn't say this patch "fixes" that commit.
What do you think? Should a fixes tag be added in this case?


> Also what effects are observed by
> the user when this happens on the kernel log?

I haven't spotted any differences visible to user, nor in the kernel log.

(In response to Brian's comment:)
>> However, as per documentation, atomic_add_unless() returns _zero_
>> if the atomic value was originally equal to the specified *unsless* value.
>>
> Nit:                                                         unless

Thanks very much for feedback. Since it's my very first upstream
commit-proposal,
I expected that some polish would be needed.


> It might be worth pointing out in the commit log that currently isolated
> buffers end up right back on the LRU once they are released, because
> ->b_lru_ref remains elevated. Therefore, this patch essentially fixes
> that circuitous route by leaving them on the LRU as originally intended.
> Otherwise this looks Ok to me:

So the final commit message could be:
~~~
Currently the xfs_buftarg_isolate() is causing an xfs_buffer
with zero b_lru_ref, to take another trip around LRU, while
isolating buffers with non-zero b_lru_ref.

Additionally those isolated buffers end up right back on the LRU
once they are released, because ->b_lru_ref remains elevated.

Fix that circuitous route by leaving them on the LRU
as originally intended.

>> Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
> Reviewed-by: Brian Foster <bfoster@redhat.com>
> ---

Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Darrick J. Wong]

On Mon, Mar 05, 2018 at 11:19:46AM +0100, Vratislav Bendel wrote:
> (In response to Luis' comment:)
> > Can you add a respective Fixes: tag?
> 
> It was apparently present since LRU was added to xfs buffer cache via:
> commit 430cbeb86fdcbbdabea7d4aa65307de8de425350
> [xfs: add a lru to the XFS buffer cache]
> 
> But I wouldn't say this patch "fixes" that commit.
> What do you think? Should a fixes tag be added in this case?
> 
> 
> > Also what effects are observed by
> > the user when this happens on the kernel log?
> 
> I haven't spotted any differences visible to user, nor in the kernel log.
> 
> (In response to Brian's comment:)
> >> However, as per documentation, atomic_add_unless() returns _zero_
> >> if the atomic value was originally equal to the specified *unsless* value.
> >>
> > Nit:                                                         unless
> 
> Thanks very much for feedback. Since it's my very first upstream
> commit-proposal,
> I expected that some polish would be needed.
> 
> 
> > It might be worth pointing out in the commit log that currently isolated
> > buffers end up right back on the LRU once they are released, because
> > ->b_lru_ref remains elevated. Therefore, this patch essentially fixes
> > that circuitous route by leaving them on the LRU as originally intended.
> > Otherwise this looks Ok to me:
> 
> So the final commit message could be:
> ~~~
> Currently the xfs_buftarg_isolate() is causing an xfs_buffer

"Due to an inverted logic mistake in xfs_buftarg_isolate()..."?

> with zero b_lru_ref, to take another trip around LRU, while

                   ^^^^ no need for this comma

> isolating buffers with non-zero b_lru_ref.
> 
> Additionally those isolated buffers end up right back on the LRU
> once they are released, because ->b_lru_ref remains elevated.
> 
> Fix that circuitous route by leaving them on the LRU
> as originally intended.

Otherwise this seems fine to me; can you please resend the patch w/
updated change log and reviewed-by tags?

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> 
> >> Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
> > Reviewed-by: Brian Foster <bfoster@redhat.com>
> > ---
> --
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> More majordomo info at  http://vger.kernel.org/majordomo-info.html

[PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Vratislav Bendel]

Due to an inverted logic mistake in xfs_buftarg_isolate()
the xfs_buffers with zero b_lru_ref will take another trip
around LRU, while isolating buffers with non-zero b_lru_ref.

Additionally those isolated buffers end up right back on the LRU
once they are released, because b_lru_ref remains elevated.

Fix that circuitous route by leaving them on the LRU
as originally intended.

Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 fs/xfs/xfs_buf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index d1da2ee9e6db..ac669a10c62f 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -1708,7 +1708,7 @@ xfs_buftarg_isolate(
 	 * zero. If the value is already zero, we need to reclaim the
 	 * buffer, otherwise it gets another trip through the LRU.
 	 */
-	if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
+	if (atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
 		spin_unlock(&bp->b_lock);
 		return LRU_ROTATE;
 	}
-- 
2.14.3

Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Darrick J. Wong]

On Tue, Mar 06, 2018 at 11:26:38AM +0100, Vratislav Bendel wrote:
> Due to an inverted logic mistake in xfs_buftarg_isolate()
> the xfs_buffers with zero b_lru_ref will take another trip
> around LRU, while isolating buffers with non-zero b_lru_ref.
> 
> Additionally those isolated buffers end up right back on the LRU
> once they are released, because b_lru_ref remains elevated.
> 
> Fix that circuitous route by leaving them on the LRU
> as originally intended.
> 
> Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
> Reviewed-by: Brian Foster <bfoster@redhat.com>
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

Looks ok, tests ok...
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/xfs/xfs_buf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index d1da2ee9e6db..ac669a10c62f 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -1708,7 +1708,7 @@ xfs_buftarg_isolate(
>  	 * zero. If the value is already zero, we need to reclaim the
>  	 * buffer, otherwise it gets another trip through the LRU.
>  	 */
> -	if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
> +	if (atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
>  		spin_unlock(&bp->b_lock);
>  		return LRU_ROTATE;
>  	}
> -- 
> 2.14.3
> 
> --
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Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>