On Mon, Jan 31, 2022 at 03:03:53PM +1100, NeilBrown wrote:
> - .writepage to return AOP_WRITEPAGE_ACTIVATE if WB_SYNC_NONE
> and the flag is set.
Is this actually useful? I ask because Dave Chinner believes
the call to ->writepage in vmscan to be essentially unused.
See commit 21b4ee7029c9, and I had a followup discussion with him
on IRC:
<willy> dchinner: did you gather any stats on how often ->writepage was
being called by pageout() before "xfs: drop ->writepage completely"
was added?
<dchinner> willy: Never saw it on XFS in 3 years in my test environment...
<dchinner> I don't ever recall seeing the memory reclaim guards we put on
->writepage in XFS ever firing - IIRC they'd been there for the best
part of a decade.
<willy> not so much the WARN_ON firing but the case where it actually calls
iomap_writepage
<dchinner> willy: I mean both - I was running with a local patch that warned
on writepage for a long time, regardless of where it was called from.
I can believe things are different for a network filesystem, or maybe
XFS does background writeback better than other filesystems, but it
would be intriguing to be able to get rid of ->writepage altogether
(or at least from pageout(); migrate.c may be a thornier proposition).
On Mon, 31 Jan 2022, Matthew Wilcox wrote:
> On Mon, Jan 31, 2022 at 03:03:53PM +1100, NeilBrown wrote:
> > - .writepage to return AOP_WRITEPAGE_ACTIVATE if WB_SYNC_NONE
> > and the flag is set.
>
> Is this actually useful? I ask because Dave Chinner believes
> the call to ->writepage in vmscan to be essentially unused.
He would be wrong ... unless "essentially" means "mostly" rather than
"totally".
swap-out to NFS results in that ->writepage call.
Of course swap_writepage ignores sync_mode, so this might not be
entirely relevant.
But the main point of the patch is not to return AOP_WRITEPAGE_ACTIVATE
to vmscan. It is to avoid writing at all when WB_SYNC_NONE and
congested. e.g. for POSIX_FADV_DONTNEED
It is also to allow the removal of congestion tracking with minimal
changes to behaviour.
If I end up changing some dead code into different dead code, I really
don't care. I'm not here to clean up all dead code - only the dead code
specifically related to congestion.
NeilBrown
> See commit 21b4ee7029c9, and I had a followup discussion with him
> on IRC:
>
> <willy> dchinner: did you gather any stats on how often ->writepage was
> being called by pageout() before "xfs: drop ->writepage completely"
> was added?
> <dchinner> willy: Never saw it on XFS in 3 years in my test environment...
> <dchinner> I don't ever recall seeing the memory reclaim guards we put on
> ->writepage in XFS ever firing - IIRC they'd been there for the best
> part of a decade.
> <willy> not so much the WARN_ON firing but the case where it actually calls
> iomap_writepage
> <dchinner> willy: I mean both - I was running with a local patch that warned
> on writepage for a long time, regardless of where it was called from.
>
> I can believe things are different for a network filesystem, or maybe
> XFS does background writeback better than other filesystems, but it
> would be intriguing to be able to get rid of ->writepage altogether
> (or at least from pageout(); migrate.c may be a thornier proposition).
>
>
On Mon, Jan 31, 2022 at 03:55:22PM +1100, NeilBrown wrote:
> On Mon, 31 Jan 2022, Matthew Wilcox wrote:
> > On Mon, Jan 31, 2022 at 03:03:53PM +1100, NeilBrown wrote:
> > > - .writepage to return AOP_WRITEPAGE_ACTIVATE if WB_SYNC_NONE
> > > and the flag is set.
> >
> > Is this actually useful? I ask because Dave Chinner believes
> > the call to ->writepage in vmscan to be essentially unused.
>
> He would be wrong ... unless "essentially" means "mostly" rather than
> "totally".
> swap-out to NFS results in that ->writepage call.
For writes, SWP_FS_OPS uses ->direct_IO, not ->writepage. Confused.
> Of course swap_writepage ignores sync_mode, so this might not be
> entirely relevant.
>
> But the main point of the patch is not to return AOP_WRITEPAGE_ACTIVATE
> to vmscan. It is to avoid writing at all when WB_SYNC_NONE and
> congested. e.g. for POSIX_FADV_DONTNEED
> It is also to allow the removal of congestion tracking with minimal
> changes to behaviour.
>
> If I end up changing some dead code into different dead code, I really
> don't care. I'm not here to clean up all dead code - only the dead code
> specifically related to congestion.
>
> NeilBrown
>
>
> > See commit 21b4ee7029c9, and I had a followup discussion with him
> > on IRC:
> >
> > <willy> dchinner: did you gather any stats on how often ->writepage was
> > being called by pageout() before "xfs: drop ->writepage completely"
> > was added?
> > <dchinner> willy: Never saw it on XFS in 3 years in my test environment...
> > <dchinner> I don't ever recall seeing the memory reclaim guards we put on
> > ->writepage in XFS ever firing - IIRC they'd been there for the best
> > part of a decade.
> > <willy> not so much the WARN_ON firing but the case where it actually calls
> > iomap_writepage
> > <dchinner> willy: I mean both - I was running with a local patch that warned
> > on writepage for a long time, regardless of where it was called from.
> >
> > I can believe things are different for a network filesystem, or maybe
> > XFS does background writeback better than other filesystems, but it
> > would be intriguing to be able to get rid of ->writepage altogether
> > (or at least from pageout(); migrate.c may be a thornier proposition).
> >
> >