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

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2024-03-07 at 22:41 +1100, NeilBrown wrote:
> 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!
> 

Thanks Neil,

I missed that bit about the sync_mode. I sent a R-b for your other patch
too.

Cheers,
Jeff


> > 
> > 
> > > 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>
> > 
> 

-- 
Jeff Layton <jlayton@kernel.org>