Messages in this thread | | | Date | Wed, 28 Jun 2023 09:33:19 +0800 | Subject | Re: [PATCH v2 09/12] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs | From | Guoqing Jiang <> |
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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
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