The bdi congestion tracking in not widely used and will be removed.
NFS is one of a small number of filesystems that uses it, setting just
the async (write) congestion flag at what it determines are appropriate
times.
The only remaining effect of the async flag is to cause (some)
WB_SYNC_NONE writes to be skipped.
So instead of setting the flag, set an internal flag and change:
- .writepages to do nothing if WB_SYNC_NONE and the flag is set
- .writepage to return AOP_WRITEPAGE_ACTIVATE if WB_SYNC_NONE
and the flag is set.
The writepages change causes a behavioural change in that pageout() can
now return PAGE_ACTIVATE instead of PAGE_KEEP, so SetPageActive() will
be called on the page which (I think) wil further delay the next attempt
at writeout. This might be a good thing.
Signed-off-by: NeilBrown <[email protected]>
---
fs/nfs/write.c | 12 ++++++++++--
include/linux/nfs_fs_sb.h | 1 +
2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 987a187bd39a..b7c6721dd36d 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -417,7 +417,7 @@ static void nfs_set_page_writeback(struct page *page)
if (atomic_long_inc_return(&nfss->writeback) >
NFS_CONGESTION_ON_THRESH)
- set_bdi_congested(inode_to_bdi(inode), BLK_RW_ASYNC);
+ nfss->write_congested = 1;
}
static void nfs_end_page_writeback(struct nfs_page *req)
@@ -433,7 +433,7 @@ static void nfs_end_page_writeback(struct nfs_page *req)
end_page_writeback(req->wb_page);
if (atomic_long_dec_return(&nfss->writeback) < NFS_CONGESTION_OFF_THRESH)
- clear_bdi_congested(inode_to_bdi(inode), BLK_RW_ASYNC);
+ nfss->write_congested = 0;
}
/*
@@ -672,6 +672,10 @@ static int nfs_writepage_locked(struct page *page,
struct inode *inode = page_file_mapping(page)->host;
int err;
+ if (wbc->sync_mode == WB_SYNC_NONE &&
+ NFS_SERVER(inode)->write_congested)
+ return AOP_WRITEPAGE_ACTIVATE;
+
nfs_inc_stats(inode, NFSIOS_VFSWRITEPAGE);
nfs_pageio_init_write(&pgio, inode, 0,
false, &nfs_async_write_completion_ops);
@@ -719,6 +723,10 @@ int nfs_writepages(struct address_space *mapping, struct writeback_control *wbc)
int priority = 0;
int err;
+ if (wbc->sync_mode == WB_SYNC_NONE &&
+ NFS_SERVER(inode)->write_congested)
+ return 0;
+
nfs_inc_stats(inode, NFSIOS_VFSWRITEPAGES);
if (!(mntflags & NFS_MOUNT_WRITE_EAGER) || wbc->for_kupdate ||
diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
index ca0959e51e81..6aa2a200676a 100644
--- a/include/linux/nfs_fs_sb.h
+++ b/include/linux/nfs_fs_sb.h
@@ -138,6 +138,7 @@ struct nfs_server {
struct nlm_host *nlm_host; /* NLM client handle */
struct nfs_iostats __percpu *io_stats; /* I/O statistics */
atomic_long_t writeback; /* number of writeback pages */
+ unsigned int write_congested;/* flag set when writeback gets too high */
unsigned int flags; /* various flags */
/* The following are for internal use only. Also see uapi/linux/nfs_mount.h */
On Tue, 01 Feb 2022, Matthew Wilcox wrote:
> 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.
>
I shouldn't have mentioned NFS - that is an irrelevant distraction.
The "call to ->writepage in vmscan" is used, at least for swap.
For swapout it is the ->writepage from swap_aops, not the ->writepage of
any filesystem. This is swap_writepage(), and for SWP_FS_OPS that maps
to a ->direct_IO call.
Dave may well be right that the ->writepage in vmscan never calls
xfs_writepage or many others.
To get to the ->writepage of a filesystem it would need to be called
from kswapd. You would need to have no swap configured, and 90% of
memory consumed with anon pages so that the dirty_background_ratio
of 10% didn't kick off writeback. Then I would expect to kswapd to
write out to a filesystem before writeback would do it.
Nonetheless, without clear evidence to the contrary, I think it is
safest to add this test to the ->writepage function for any filesystem
which currently sets the bdi async congested flag.
Thanks,
NeilBrown