Re: [PATCH v2 09/12] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs

[Guoqing Jiang <guoqing.jiang@linux.dev>]

Hi Ojaswin,

On 6/27/23 14:51, Ojaswin Mujoo wrote:
> On Tue, Jun 06, 2023 at 10:00:57PM +0800, Guoqing Jiang wrote:
>> Hello,
>>
>> On 5/30/23 20:33, Ojaswin Mujoo wrote:
>>> Before this patch, the call stack in ext4_run_li_request is as follows:
>>>
>>>     /*
>>>      * nr = no. of BGs we want to fetch (=s_mb_prefetch)
>>>      * prefetch_ios = no. of BGs not uptodate after
>>>      * 		    ext4_read_block_bitmap_nowait()
>>>      */
>>>     next_group = ext4_mb_prefetch(sb, group, nr, prefetch_ios);
>>>     ext4_mb_prefetch_fini(sb, next_group prefetch_ios);
>>>
>>> ext4_mb_prefetch_fini() will only try to initialize buddies for BGs in
>>> range [next_group - prefetch_ios, next_group). This is incorrect since
>>> sometimes (prefetch_ios < nr), which causes ext4_mb_prefetch_fini() to
>>> incorrectly ignore some of the BGs that might need initialization. This
>>> issue is more notable now with the previous patch enabling "fetching" of
>>> BLOCK_UNINIT BGs which are marked buffer_uptodate by default.
>>>
>>> Fix this by passing nr to ext4_mb_prefetch_fini() instead of
>>> prefetch_ios so that it considers the right range of groups.
>> Thanks for the series.
>>
>>> Similarly, make sure we don't pass nr=0 to ext4_mb_prefetch_fini() in
>>> ext4_mb_regular_allocator() since we might have prefetched BLOCK_UNINIT
>>> groups that would need buddy initialization.
>> Seems ext4_mb_prefetch_fini can't be called by ext4_mb_regular_allocator
>> if nr is 0.
> Hi Guoqing,
>
> Sorry I was on vacation so didn't get a chance to reply to this sooner.
> Let me explain what I meant by that statement in the commit message.
>
> So basically, the prefetch_ios output argument is incremented whenever
> ext4_mb_prefetch() reads a block group with !buffer_uptodate(bh).
> However, for BLOCK_UNINIT BGs the buffer is marked uptodate after
> initialization and hence prefetch_ios is not incremented when such BGs
> are prefetched.
>
> This leads to nr becoming 0 due to the following line (removed in this patch):
>
> 				if (prefetch_ios == curr_ios)
> 					nr = 0;
>
> hence ext4_mb_prefetch_fini() would never pre initialise the corresponding
> buddy structures. Instead, these structures would then get initialized
> probably at a later point during the slower allocation criterias. The
> motivation of making sure the BLOCK_UNINIT BGs' buddies are pre
> initialized is so the faster allocation criterias can utilize the data
> to make better decisions.

Got it, appreciate for the detailed explanation!

Thanks,
Guoqing