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From:   Chuck Lever III <chuck.lever@oracle.com>
To:     Hugh Dickins <hughd@google.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Mark Hemment <markhemm@googlemail.com>,
        Patrice CHOTARD <patrice.chotard@foss.st.com>,
        Mikulas Patocka <mpatocka@redhat.com>,
        Lukas Czerner <lczerner@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: Regression in xfstests on tmpfs-backed NFS exports
Thread-Topic: Regression in xfstests on tmpfs-backed NFS exports
Thread-Index: AQHYSdpNL2d3Xru4B0OzHZJ1LCngEazjlm2AgAE//ICAADMPAIAAFfQA
Date:   Thu, 7 Apr 2022 23:45:31 +0000
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> On Apr 7, 2022, at 6:26 PM, Hugh Dickins <hughd@google.com> wrote:
>=20
> On Thu, 7 Apr 2022, Chuck Lever III wrote:
>>> On Apr 6, 2022, at 8:18 PM, Hugh Dickins <hughd@google.com> wrote:
>>>=20
>>> But I can sit here and try to guess.  I notice fs/nfsd checks
>>> file->f_op->splice_read, and employs fallback if not available:
>>> if you have time, please try rerunning those xfstests on an -rc1
>>> kernel, but with mm/shmem.c's .splice_read line commented out.
>>> My guess is that will then pass the tests, and we shall know more.
>>=20
>> This seemed like the most probative next step, so I commented
>> out the .splice_read call-out in mm/shmem.c and ran the tests
>> again. Yes, that change enables the fsx-related tests to pass
>> as expected.
>=20
> Great, thank you for trying that.
>=20
>>=20
>>> What could be going wrong there?  I've thought of two possibilities.
>>> A minor, hopefully easily fixed, issue would be if fs/nfsd has
>>> trouble with seeing the same page twice in a row: since tmpfs is
>>> now using the ZERO_PAGE(0) for all pages of a hole, and I think I
>>> caught sight of code which looks to see if the latest page is the
>>> same as the one before.  It's easy to imagine that might go wrong.
>>=20
>> Are you referring to this function in fs/nfsd/vfs.c ?
>=20
> I think that was it, didn't pay much attention.

This code seems to have been the issue. I added a little test
to see if @page pointed to ZERO_PAGE(0) and now the tests
pass as expected.


>> 847 static int
>> 848 nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *=
buf,
>> 849                   struct splice_desc *sd)
>> 850 {
>> 851         struct svc_rqst *rqstp =3D sd->u.data;
>> 852         struct page **pp =3D rqstp->rq_next_page;
>> 853         struct page *page =3D buf->page;
>> 854=20
>> 855         if (rqstp->rq_res.page_len =3D=3D 0) {
>> 856                 svc_rqst_replace_page(rqstp, page);
>> 857                 rqstp->rq_res.page_base =3D buf->offset;
>> 858         } else if (page !=3D pp[-1]) {
>> 859                 svc_rqst_replace_page(rqstp, page);
>> 860         }
>> 861         rqstp->rq_res.page_len +=3D sd->len;
>> 862=20
>> 863         return sd->len;
>> 864 }
>>=20
>> rq_next_page should point to the first unused element of
>> rqstp->rq_pages, so IIUC that check is looking for the
>> final page that is part of the READ payload.
>>=20
>> But that does suggest that if page -> ZERO_PAGE and so does
>> pp[-1], then svc_rqst_replace_page() would not be invoked.
>=20
> I still haven't studied the logic there: Mark's input made it clear
> that it's just too risky for tmpfs to pass back ZERO_PAGE repeatedly,
> there could be expectations of uniqueness in other places too.

I can't really attest to Mark's comment, but...

After studying nfsd_splice_actor() I can't see any reason
except cleverness and technical debt for this particular
check. I have a patch that removes the check and simplifies
this function that I'm testing now -- it seems to be a
reasonable clean-up whether you keep 56a8c8eb1eaf or
choose to revert it.


>>> A more difficult issue would be, if fsx is racing writes and reads,
>>> in a way that it can guarantee the correct result, but that correct
>>> result is no longer delivered: because the writes go into freshly
>>> allocated tmpfs cache pages, while reads are still delivering
>>> stale ZERO_PAGEs from the pipe.  I'm hazy on the guarantees there.
>>>=20
>>> But unless someone has time to help out, we're heading for a revert.
>=20
> We might be able to avoid that revert, and go the whole way to using
> iov_iter_zero() instead.  But the significant slowness of clear_user()
> relative to copy to user, on x86 at least, does ask for a hybrid.
>=20
> Suggested patch below, on top of 5.18-rc1, passes my own testing:
> but will it pass yours?  It seems to me safe, and as fast as before,
> but we don't know yet if this iov_iter_zero() works right for you.
> Chuck, please give it a go and let us know.
>=20
> (Don't forget to restore mm/shmem.c's .splice_read first!  And if
> this works, I can revert mm/filemap.c's SetPageUptodate(ZERO_PAGE(0))
> in the same patch, fixing the other regression, without recourse to
> #ifdefs or arch mods.)

Sure, I will try this out first thing tomorrow.

One thing that occurs to me is that for NFS/RDMA, having a
page full of zeroes that is already DMA-mapped would be a
nice optimization on the sender side (on the client for an
NFS WRITE and on the server for an NFS READ). The transport
would have to set up a scatter-gather list containing a
bunch of entries that reference the same page...

</musing>


> Thanks!
> Hugh
>=20
> --- 5.18-rc1/mm/shmem.c
> +++ linux/mm/shmem.c
> @@ -2513,7 +2513,6 @@ static ssize_t shmem_file_read_iter(struct kiocb *i=
ocb, struct iov_iter *to)
> 		pgoff_t end_index;
> 		unsigned long nr, ret;
> 		loff_t i_size =3D i_size_read(inode);
> -		bool got_page;
>=20
> 		end_index =3D i_size >> PAGE_SHIFT;
> 		if (index > end_index)
> @@ -2570,24 +2569,34 @@ static ssize_t shmem_file_read_iter(struct kiocb =
*iocb, struct iov_iter *to)
> 			 */
> 			if (!offset)
> 				mark_page_accessed(page);
> -			got_page =3D true;
> +			/*
> +			 * Ok, we have the page, and it's up-to-date, so
> +			 * now we can copy it to user space...
> +			 */
> +			ret =3D copy_page_to_iter(page, offset, nr, to);
> +			put_page(page);
> +
> +		} else if (iter_is_iovec(to)) {
> +			/*
> +			 * Copy to user tends to be so well optimized, but
> +			 * clear_user() not so much, that it is noticeably
> +			 * faster to copy the zero page instead of clearing.
> +			 */
> +			ret =3D copy_page_to_iter(ZERO_PAGE(0), offset, nr, to);
> 		} else {
> -			page =3D ZERO_PAGE(0);
> -			got_page =3D false;
> +			/*
> +			 * But submitting the same page twice in a row to
> +			 * splice() - or others? - can result in confusion:
> +			 * so don't attempt that optimization on pipes etc.
> +			 */
> +			ret =3D iov_iter_zero(nr, to);
> 		}
>=20
> -		/*
> -		 * Ok, we have the page, and it's up-to-date, so
> -		 * now we can copy it to user space...
> -		 */
> -		ret =3D copy_page_to_iter(page, offset, nr, to);
> 		retval +=3D ret;
> 		offset +=3D ret;
> 		index +=3D offset >> PAGE_SHIFT;
> 		offset &=3D ~PAGE_MASK;
>=20
> -		if (got_page)
> -			put_page(page);
> 		if (!iov_iter_count(to))
> 			break;
> 		if (ret < nr) {

--
Chuck Lever