Re: [PATCH] NFS: filelayout should use nfs_generic_pg_test

[Benny Halevy <bhalevy@panasas.com>]

On 2011-06-01 21:07, Trond Myklebust wrote:
> On Wed, 2011-06-01 at 17:51 +0300, Benny Halevy wrote: 
>> I think the following should work:
>>
>> Benny
>>
>> git diff --stat -p -M
>>  fs/nfs/nfs4filelayout.c |   10 ++++++++++
>>  1 files changed, 10 insertions(+), 0 deletions(-)
>>
>> diff --git a/fs/nfs/nfs4filelayout.c b/fs/nfs/nfs4filelayout.c
>> index 4269088..9f1d445 100644
>> --- a/fs/nfs/nfs4filelayout.c
>> +++ b/fs/nfs/nfs4filelayout.c
>> @@ -661,6 +661,16 @@ filelayout_pg_test(struct nfs_pageio_descriptor
>> *pgio, struct nfs_page *prev,
>>  	u64 p_stripe, r_stripe;
>>  	u32 stripe_unit;
>>
>> +	/*
>> +	 * FIXME: ideally we should be able to coalesce all requests
>> +	 * that are not block boundary aligned, but currently this
>> +	 * is problematic for the case of bsize < PAGE_CACHE_SIZE,
>> +	 * since nfs_flush_multi and nfs_pagein_multi assume you
>> +	 * can have only one struct nfs_page.
>> +	 */
>> +	if (desc->pg_bsize < PAGE_SIZE)
>> +		return 0;
>> +
>>  	if (!pnfs_generic_pg_test(pgio, prev, req))
>>  		return 0;
> 
> So, there are several things that bother me about pnfs_generic_pg_test()
> too now that I'm looking more closely at it:
> 
>      1. If the intention is to coalesce 'prev' and 'req', shouldn't we
>         be asking for a layout with req_offset(prev) instead of
>         req_offset(req)? 
>      2. If we're only requesting a layout of length pg_count, don't we
>         still need to test the layout length that the server actually
>         returned before we can allow the coalescing? 
>      3. if (!pgio->lseg), shouldn't we be returning an error of some
>         sort? Right now we're returning 'true', and allowing the
>         coalesce to occur. 
>      4. Furthermore, shouldn't that test guarding the
>         pnfs_update_layout() call rather be an 'if (pgio->pg_lseg ==
>         NULL)' instead of looking at the values of pg_count and
>         prev->wb_bytes?
> 

or rather we get the layout for the first page in
nfs_pageio_do_add_request when desc->pg_count == 0?

Then, this lseg would be useful for nfs_flush_multi
if we failed to coalesce, or we failed to get a layout
altogether we go the nfs path and can reset pg_test to
nfs_generic_pg_test.

Otherwise I agree with your assertions above.

Benny