Re: [PATCH stable 6.6 and 6.7] NFS: Fix data corruption caused by congestion.

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2024-03-06 at 08:42 -0500, 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;
> 

Sorry, I meant to say:

The only caller of nfs_writepages_locked is nfs_wb_folio, and I don't
see where it redirties the folio. Why don't we need this in v6.8?


> 
> > Reported-and-tested-by: Jacek Tomaka <Jacek.Tomaka@poczta.fm>
> > Fixes: 6df25e58532b ("nfs: remove reliance on bdi congestion")
> > Signed-off-by: NeilBrown <neilb@suse.de>
> > ---
> >  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@kernel.org>