Re: [PATCH v1 19/42] SUNRPC: Fix xdr_get_next_encode_buffer() page boundary handling

From: Chuck Lever <chuck.lever@oracle.com>
To: Bruce Fields <bfields@fieldses.org>
Cc: Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH v1 19/42] SUNRPC: Fix xdr_get_next_encode_buffer() page boundary handling
Date: Wed, 3 Mar 2021 15:43:28 +0000	[thread overview]
Message-ID: <592A34CB-C178-4272-8905-F3BA95BCE299@oracle.com> (raw)
In-Reply-To: <20210302221130.GG3400@fieldses.org>

Hi Bruce-

Thanks for your careful review of this series!

> On Mar 2, 2021, at 5:11 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> 
> On Mon, Mar 01, 2021 at 10:17:13AM -0500, Chuck Lever wrote:
>> The description of commit 2825a7f90753 ("nfsd4: allow encoding
>> across page boundaries") states:
>> 
>>> Also we can't handle a new operation starting close to the end of
>>> a page.
>> 
>> But does not detail why this is the case.
> 
> That wasn't every helpful of me, sorry.
> 
>> Subtracting the scratch buffer's "shift" value from the remaining
>> stream space seems to make reserving space close to the end of the
>> buf->pages array reliable.
> 
> So, why is that?
> 
> Thinking this through:
> 
> When somebody asks for a buffer that would straddle a page boundary,
> with frag1bytes at the end of this page and frag2bytes at the start of
> the next page, we instead give them a buffer starting at start of the
> next page.  That gives them a nice contiguous buffer to write into.
> When they're done using it, we fix things up by copying what they wrote
> back to where it should be.
> 
> That means we're temporarily wasting frag1bytes of space.  So, I don't
> know, maybe that's the logic behind subtracing frag1bytes from
> space_left.
> 
> It means you may end up with xdr->end frag1bytes short of the next page.
> I'm not sure that's right.

Why would that not be OK? the next call to xdr_get_next_encode_buffer()
should do the right thing and bounce the new encoded data from the
next page into this one again.

So far I have not encountered any problems. Would such a problem show
up with some frequency under normal use, or would it be especially
subtle?

> --b.
> 
> 
>> 
>> This change is needed to make entry encoding with struct xdr_stream,
>> introduced in a subsequent patch, work correctly when it approaches
>> the end of the dirlist buffer.
>> 
>> Fixes: 2825a7f90753 ("nfsd4: allow encoding across page boundaries")
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>> ---
>> net/sunrpc/xdr.c |    2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
>> index 3964ff74ee51..043b67229792 100644
>> --- a/net/sunrpc/xdr.c
>> +++ b/net/sunrpc/xdr.c
>> @@ -978,7 +978,7 @@ static __be32 *xdr_get_next_encode_buffer(struct xdr_stream *xdr,
>> 	 * shifted this one back:
>> 	 */
>> 	xdr->p = (void *)p + frag2bytes;
>> -	space_left = xdr->buf->buflen - xdr->buf->len;
>> +	space_left = xdr->buf->buflen - xdr->buf->len - frag1bytes;
>> 	xdr->end = (void *)p + min_t(int, space_left, PAGE_SIZE);
>> 	xdr->buf->page_len += frag2bytes;
>> 	xdr->buf->len += nbytes;
>> 

--
Chuck Lever