On Thu, 07 Mar 2024, Jeff Layton wrote:
> On Wed, 2024-02-28 at 10:23 +1100, NeilBrown wrote:
> > when AOP_WRITEPAGE_ACTIVATE is returned (as NFS does when it detects
> > congestion) it is important that the folio is redirtied.
> > nfs_writepage_locked() doesn't do this, so files can become corrupted as
> > writes can be lost.
> >
> > Note that this is not needed in v6.8 as AOP_WRITEPAGE_ACTIVATE cannot be
> > returned. It is needed for kernels v5.18..v6.7. Prior to 6.3 the patch
> > is different as it needs to mention "page", not "folio".
> >
>
> Neil, I have a question about the above statement. In Linus's tree as of
> this morning (v6.8-rc7-ish), it does this in nfs_writepages_locked:
>
> if (wbc->sync_mode == WB_SYNC_NONE &&
> NFS_SERVER(inode)->write_congested)
> return AOP_WRITEPAGE_ACTIVATE;
>
> The only caller of nfs_writepages_locked, and I don't see where it
> redirties the page. Why don't we need this in v6.8?
You are right - it doesn't redirty anything. But there is no bug
here....
I didn't see it at first either, but the only caller of
nfs_writepage_locked() is nfs_wb_folio() (as you say) and that always
passes a wbc with .sync_mode = WB_SYNC_ALL. So sync_mode is never
WB_SYNC_NODE and the code snippet you included above is dead code. I've
already posted a patch to Trond and Anna to remove that code.
Thanks for the review!
NeilBrown
>
>
> > Reported-and-tested-by: Jacek Tomaka <Jacek.Tomaka@xxxxxxxxx>
> > Fixes: 6df25e58532b ("nfs: remove reliance on bdi congestion")
> > Signed-off-by: NeilBrown <neilb@xxxxxxx>
> > ---
> > fs/nfs/write.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> > index b664caea8b4e..9e345d3c305a 100644
> > --- a/fs/nfs/write.c
> > +++ b/fs/nfs/write.c
> > @@ -668,8 +668,10 @@ static int nfs_writepage_locked(struct folio *folio,
> > int err;
> >
> > if (wbc->sync_mode == WB_SYNC_NONE &&
> > - NFS_SERVER(inode)->write_congested)
> > + NFS_SERVER(inode)->write_congested) {
> > + folio_redirty_for_writepage(wbc, folio);
> > return AOP_WRITEPAGE_ACTIVATE;
> > + }
> >
> > nfs_inc_stats(inode, NFSIOS_VFSWRITEPAGE);
> > nfs_pageio_init_write(&pgio, inode, 0, false,
>
> --
> Jeff Layton <jlayton@xxxxxxxxxx>
>
