linux-ext4 - Re: [PATCH v2 09/12] ext4: Ensure ext4_mb_prefetch

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Date:   Tue, 27 Jun 2023 12:21:38 +0530
From:   Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To:     Guoqing Jiang <guoqing.jiang@...ux.dev>
Cc:     linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>,
        Kemeng Shi <shikemeng@...weicloud.com>,
        Ritesh Harjani <ritesh.list@...il.com>
Subject: Re: [PATCH v2 09/12] ext4: Ensure ext4_mb_prefetch_fini() is called
 for all prefetched BGs

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.

Regards,
ojaswin

> 
> https://elixir.bootlin.com/linux/v6.4-rc5/source/fs/ext4/mballoc.c#L2816
> 
> Am I miss something?
> 
> Thanks,
> Guoqing
>