Re: parts of pages on NFS being replaced by swaths of NULs

[Trond Myklebust <trondmy@kernel.org>]

On Tue, 2025-08-19 at 13:10 -0400, Jeff Layton wrote:
> On Sat, 2025-08-16 at 07:51 -0700, Trond Myklebust wrote:
> > On Sat, 2025-08-16 at 09:01 -0400, Jeff Layton wrote:
> > > 
> > > I finally caught something concrete today. I had the attached
> > > bpftrace
> > > script running while running the reproducer on a dozen or so
> > > machines,
> > > and it detected a hole in some data being written:
> > > 
> > > -------------8<---------------
> > > Attached 2 probes
> > > Missing nfs_page: ino=10122173116 idx=2 flags=0x15ffff0000000029
> > > Hole: ino=10122173116 idx=3 off=10026 size=2262
> > > Prev folio: idx=2 flags=0x15ffff0000000028 pgbase=0 bytes=4096
> > > req=0
> > > prevreq=0xffff8955b2f55980
> > > -------------8<---------------
> > > 
> > > What this tells us is that the page at idx=2 got submitted to
> > > nfs_do_writepage() (so it was marked dirty in the pagecache), but
> > > when
> > > it got there, folio->private was NULL and it was ignored.
> > > 
> > > The kernel in this case is based on v6.9, so it's (just) pre-
> > > large-
> > > folio support. It has a fair number of NFS patches, but not much
> > > to
> > > this portion of the code. Most of them are are containerization
> > > fixes.
> > > 
> > > I'm looking askance at nfs_inode_remove_request(). It does this:
> > > 
> > >         if (nfs_page_group_sync_on_bit(req, PG_REMOVE)) {
> > >                 struct folio *folio = nfs_page_to_folio(req-
> > > > wb_head);
> > >                 struct address_space *mapping = folio->mapping;
> > > 
> > >                 spin_lock(&mapping->i_private_lock);
> > >                 if (likely(folio)) {
> > >                         folio->private = NULL;
> > >                         folio_clear_private(folio);
> > >                         clear_bit(PG_MAPPED, &req->wb_head-
> > > > wb_flags);
> > >                 }
> > >                 spin_unlock(&mapping->i_private_lock);
> > >         }
> > > 
> > > If nfs_page_group_sync_on_bit() returns true, then the nfs_page
> > > gets
> > > detached from the folio. Meanwhile, if a new write request comes
> > > in
> > > just after that, nfs_lock_and_join_requests() will call
> > > nfs_cancel_remove_inode() to try to "cancel" PG_REMOVE:
> > > 
> > > static int
> > > nfs_cancel_remove_inode(struct nfs_page *req, struct inode
> > > *inode)
> > > {
> > >         int ret;
> > > 
> > >         if (!test_bit(PG_REMOVE, &req->wb_flags))
> > >                 return 0;
> > >         ret = nfs_page_group_lock(req);
> > >         if (ret)
> > >                 return ret;
> > >         if (test_and_clear_bit(PG_REMOVE, &req->wb_flags))
> > >                 nfs_page_set_inode_ref(req, inode);
> > >         nfs_page_group_unlock(req);                          
> > >         return 0;                                    
> > > }                     
> > > 
> > > ...but that does not reattach the nfs_page to the folio. Should
> > > it?
> > >                         
> > 
> > That's not sufficient AFAICS. Does the following patch work?
> > 
> > 8<------------------------------------------------------------
> > From fc9690dda01f001c6cd11665701394da8ebba1ab Mon Sep 17 00:00:00
> > 2001
> > Message-ID:
> > <fc9690dda01f001c6cd11665701394da8ebba1ab.1755355810.git.trond.mykl
> > ebust@hammerspace.com>
> > From: Trond Myklebust <trond.myklebust@hammerspace.com>
> > Date: Sat, 16 Aug 2025 07:25:20 -0700
> > Subject: [PATCH] NFS: Fix a race when updating an existing write
> > 
> > After nfs_lock_and_join_requests() tests for whether the request is
> > still attached to the mapping, nothing prevents a call to
> > nfs_inode_remove_request() from succeeding until we actually lock
> > the
> > page group.
> > The reason is that whoever called nfs_inode_remove_request()
> > doesn't
> > necessarily have a lock on the page group head.
> > 
> > So in order to avoid races, let's take the page group lock earlier
> > in
> > nfs_lock_and_join_requests(), and hold it across the removal of the
> > request in nfs_inode_remove_request().
> > 
> > Reported-by: Jeff Layton <jlayton@kernel.org>
> > Fixes: c3f2235782c3 ("nfs: fold nfs_folio_find_and_lock_request
> > into nfs_lock_and_join_requests")
> > Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> > ---
> >  fs/nfs/pagelist.c        |  9 +++++----
> >  fs/nfs/write.c           | 29 ++++++++++-------------------
> >  include/linux/nfs_page.h |  1 +
> >  3 files changed, 16 insertions(+), 23 deletions(-)
> > 
> > diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
> > index 11968dcb7243..6e69ce43a13f 100644
> > --- a/fs/nfs/pagelist.c
> > +++ b/fs/nfs/pagelist.c
> > @@ -253,13 +253,14 @@ nfs_page_group_unlock(struct nfs_page *req)
> >  	nfs_page_clear_headlock(req);
> >  }
> >  
> > -/*
> > - * nfs_page_group_sync_on_bit_locked
> > +/**
> > + * nfs_page_group_sync_on_bit_locked - Test if all requests have
> > @bit set
> > + * @req: request in page group
> > + * @bit: PG_* bit that is used to sync page group
> >   *
> >   * must be called with page group lock held
> >   */
> > -static bool
> > -nfs_page_group_sync_on_bit_locked(struct nfs_page *req, unsigned
> > int bit)
> > +bool nfs_page_group_sync_on_bit_locked(struct nfs_page *req,
> > unsigned int bit)
> >  {
> >  	struct nfs_page *head = req->wb_head;
> >  	struct nfs_page *tmp;
> > diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> > index fa5c41d0989a..8b7c04737967 100644
> > --- a/fs/nfs/write.c
> > +++ b/fs/nfs/write.c
> > @@ -153,20 +153,10 @@ nfs_page_set_inode_ref(struct nfs_page *req,
> > struct inode *inode)
> >  	}
> >  }
> >  
> > -static int
> > -nfs_cancel_remove_inode(struct nfs_page *req, struct inode *inode)
> > +static void nfs_cancel_remove_inode(struct nfs_page *req, struct
> > inode *inode)
> >  {
> > -	int ret;
> > -
> > -	if (!test_bit(PG_REMOVE, &req->wb_flags))
> > -		return 0;
> > -	ret = nfs_page_group_lock(req);
> > -	if (ret)
> > -		return ret;
> >  	if (test_and_clear_bit(PG_REMOVE, &req->wb_flags))
> >  		nfs_page_set_inode_ref(req, inode);
> > -	nfs_page_group_unlock(req);
> > -	return 0;
> >  }
> >  
> >  /**
> > @@ -585,19 +575,18 @@ static struct nfs_page
> > *nfs_lock_and_join_requests(struct folio *folio)
> >  		}
> >  	}
> >  
> > +	ret = nfs_page_group_lock(head);
> > +	if (ret < 0)
> > +		goto out_unlock;
> > +
> >  	/* Ensure that nobody removed the request before we locked
> > it */
> >  	if (head != folio->private) {
> > +		nfs_page_group_unlock(head);
> >  		nfs_unlock_and_release_request(head);
> >  		goto retry;
> >  	}
> >  
> > -	ret = nfs_cancel_remove_inode(head, inode);
> > -	if (ret < 0)
> > -		goto out_unlock;
> > -
> > -	ret = nfs_page_group_lock(head);
> > -	if (ret < 0)
> > -		goto out_unlock;
> > +	nfs_cancel_remove_inode(head, inode);
> >  
> >  	/* lock each request in the page group */
> >  	for (subreq = head->wb_this_page;
> > @@ -786,7 +775,8 @@ static void nfs_inode_remove_request(struct
> > nfs_page *req)
> >  {
> >  	struct nfs_inode *nfsi = NFS_I(nfs_page_to_inode(req));
> >  
> > -	if (nfs_page_group_sync_on_bit(req, PG_REMOVE)) {
> > +	nfs_page_group_lock(req);
> > +	if (nfs_page_group_sync_on_bit_locked(req, PG_REMOVE)) {
> >  		struct folio *folio = nfs_page_to_folio(req-
> > >wb_head);
> >  		struct address_space *mapping = folio->mapping;
> >  
> > @@ -798,6 +788,7 @@ static void nfs_inode_remove_request(struct
> > nfs_page *req)
> >  		}
> >  		spin_unlock(&mapping->i_private_lock);
> >  	}
> > +	nfs_page_group_unlock(req);
> >  
> >  	if (test_and_clear_bit(PG_INODE_REF, &req->wb_flags)) {
> >  		atomic_long_dec(&nfsi->nrequests);
> > diff --git a/include/linux/nfs_page.h b/include/linux/nfs_page.h
> > index 169b4ae30ff4..9aed39abc94b 100644
> > --- a/include/linux/nfs_page.h
> > +++ b/include/linux/nfs_page.h
> > @@ -160,6 +160,7 @@ extern void nfs_join_page_group(struct nfs_page
> > *head,
> >  extern int nfs_page_group_lock(struct nfs_page *);
> >  extern void nfs_page_group_unlock(struct nfs_page *);
> >  extern bool nfs_page_group_sync_on_bit(struct nfs_page *, unsigned
> > int);
> > +extern bool nfs_page_group_sync_on_bit_locked(struct nfs_page *,
> > unsigned int);
> >  extern	int nfs_page_set_headlock(struct nfs_page *req);
> >  extern void nfs_page_clear_headlock(struct nfs_page *req);
> >  extern bool nfs_async_iocounter_wait(struct rpc_task *, struct
> > nfs_lock_context *);
> 
> I backported this patch to the kernel we've been using to reproduce
> this and have had the test running for almost 24 hours now. The
> longest
> it's taken to reproduce on this test rig is about 12 hours. So, the
> initial signs are good.
> 
> The patch also looks good to me. This one took a while to track down,
> and I needed a lot of help to set up the test rig. Can you add these?
> 
> Tested-by: Joe Quanaim <jdq@meta.com>
> Tested-by: Andrew Steffen <aksteffen@meta.com>
> Reviewed-by: Jeff Layton <jlayton@kernel.org>
> 
> Joe and Andrew spent a lot of time getting us a reproducer.
> 
> I assume we also want to send this to stable? I'm pretty sure the
> Fixes: tag is wrong. The kernel we were using didn't have that patch.
> I
> took a look at some earlier releases, and AFAICT, this bug has been
> present for a long time -- at least since v6.0 and probably well
> before.
> 

I've set
Fixes: bd37d6fce184 ("NFSv4: Convert nfs_lock_and_join_requests() to use nfs_page_find_head_request()")

on the very latest commit tags. That's about as far back as I can trace
it before going cross-eyed.
-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trondmy@kernel.org, trond.myklebust@hammerspace.com