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
