xfs_buf and buffercache/pagecache connection

[Yannis Klonatos]

???? 5/31/2010 9:04 PM, O/H Andi Kleen ??????:
> Yannis Klonatos<klonatos at ics.forth.gr>  writes:
>
>    
>>          I was looking to add a kernel hook to my system in order to
>> monitor buffer-cache hit and misses. Initially I was
>> planning to add my modifications to the __getblk(). However, i noticed
>> that XFS does not directly use the buffer-cache
>> for its pages but it seems to implement its own buffer.
>>          What I am now looking for is 1) the place where XFS checks
>> whether a page exists in its buffer or not and 2)
>> what are the possible interactions between xfs_buf and the Linux
>> kernel buffer-cache.
>>          I would appreciate any information regarding the above issues.
>>      
> The kernel does not track all accesses, e.g. through mmap.
> So you can only get misses (which is essentially IO rate and already
> accounted), but not hits.
>
> -Andi
>    

First of all thanks for your quick reply.

So, if i understand correctly, what you are saying is that it is 
basically impossible to modify xfs code
so that i get that specific information? This sounds a bit strange since 
if XFS was indeed using the
buffercache as ext3 or other fs does, the following modification would 
suffice (file fs/buffer.c):

structbuffer_head  <+code=buffer_head>  *
find_get_block  <+code=__find_get_block>(structblock_device  <+code=block_device>  *bdev  <+code=bdev>,sector_t  <+code=sector_t>  block  <+code=block>, intsize  <+code=size>) {

      	structbuffer_head  <+code=buffer_head>  *bh  <+code=bh>  =lookup_bh_lru  <+code=lookup_bh_lru>(bdev  <+code=bdev>,block  <+code=block>,size  <+code=size>);
	/* BEGIN MODIFICATION */
	if (bh) buffercache_hits++;
	else buffercache_misses++;
	/* END MODIFICATION */
         if (bh  <+code=bh>  ==NULL  <+code=NULL>) {
                bh  <+code=bh>  =__find_get_block_slow  <+code=__find_get_block_slow>(bdev  <+code=bdev>,block  <+code=block>);
                if (bh  <+code=bh>)
                        bh_lru_install  <+code=bh_lru_install>(bh  <+code=bh>);
        }
        if (bh  <+code=bh>) {
                touch_buffer  <+code=touch_buffer>(bh  <+code=bh>);
        returnbh  <+code=bh>;

}

XPORT_SYMBOL  <+code=EXPORT_SYMBOL>(__find_get_block  <+code=__find_get_block>);


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