Re: [PATCH 4/8] mm/compaction: remove stale fast_find_block flag in isolate_migratepages

[Baolin Wang]

On 8/1/2023 11:48 AM, Kemeng Shi wrote:
> on 8/1/2023 11:34 AM, Baolin Wang wrote:
> > On 8/1/2023 11:24 AM, Kemeng Shi wrote:
> > > on 8/1/2023 10:42 AM, Baolin Wang wrote:
> > > > On 7/29/2023 1:10 AM, Kemeng Shi wrote:
> > > > > In old code, we set skip to found page block in fast_find_migrateblock. So
> > > > > we use fast_find_block to avoid skip found page block from
> > > > > fast_find_migrateblock.
> > > > > In 90ed667c03fe5 ("Revert "Revert "mm/compaction: fix set skip in
> > > > > fast_find_migrateblock"""), we remove skip set in fast_find_migrateblock,
> > > > > then fast_find_block is useless.
> > > > >
> > > > > Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
> > > > > ---
> > > > >     mm/compaction.c | 12 +-----------
> > > > >     1 file changed, 1 insertion(+), 11 deletions(-)
> > > > >
> > > > > diff --git a/mm/compaction.c b/mm/compaction.c
> > > > > index ad535f880c70..09c36251c613 100644
> > > > > --- a/mm/compaction.c
> > > > > +++ b/mm/compaction.c
> > > > > @@ -1949,7 +1949,6 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
> > > > >         const isolate_mode_t isolate_mode =
> > > > >             (sysctl_compact_unevictable_allowed ? ISOLATE_UNEVICTABLE : 0) |
> > > > >             (cc->mode != MIGRATE_SYNC ? ISOLATE_ASYNC_MIGRATE : 0);
> > > > > -    bool fast_find_block;
> > > > >           /*
> > > > >          * Start at where we last stopped, or beginning of the zone as
> > > > > @@ -1961,13 +1960,6 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
> > > > >         if (block_start_pfn < cc->zone->zone_start_pfn)
> > > > >             block_start_pfn = cc->zone->zone_start_pfn;
> > > > >     -    /*
> > > > > -     * fast_find_migrateblock marks a pageblock skipped so to avoid
> > > > > -     * the isolation_suitable check below, check whether the fast
> > > > > -     * search was successful.
> > > > > -     */
> > > > > -    fast_find_block = low_pfn != cc->migrate_pfn && !cc->fast_search_fail;
> > > > > -
> > > > >         /* Only scan within a pageblock boundary */
> > > > >         block_end_pfn = pageblock_end_pfn(low_pfn);
> > > > >     @@ -1976,7 +1968,6 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
> > > > >          * Do not cross the free scanner.
> > > > >          */
> > > > >         for (; block_end_pfn <= cc->free_pfn;
> > > > > -            fast_find_block = false,
> > > > >                 cc->migrate_pfn = low_pfn = block_end_pfn,
> > > > >                 block_start_pfn = block_end_pfn,
> > > > >                 block_end_pfn += pageblock_nr_pages) {
> > > > > @@ -2007,8 +1998,7 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
> > > > >              * before making it "skip" so other compaction instances do
> > > > >              * not scan the same block.
> > > > >              */
> > > > > -        if (pageblock_aligned(low_pfn) &&
> > > > > -            !fast_find_block && !isolation_suitable(cc, page))
> > > > > +        if (pageblock_aligned(low_pfn) && !isolation_suitable(cc, page))
> > > >
> > > > I do not think so. If the pageblock is found by fast_find_migrateblock(), that means it definitely has not been set the skip flag, so there is not need to call isolation_suitable() if fast_find_block is true, right?
> > > Actually, found pageblock could be set skip as:
> > > 1. other compactor could mark this pageblock as skip after zone lock is realeased
> > > in fast_find_migrateblock.
> >
> > Yes, but your patch also can not close this race window, that means it can also be set skip flag after the isolation_suitable() validation by other compactors.
> Yes, I think it's still worth to remove a lot of fast_find_block relevant check and reduce
> code complexity with one redundant isolation_suitable which may skip some block with luck.
> > > 2. fast_find_migrateblock may uses pfn from reinit_migrate_pfn which is previously found
> > > and sacnned. It could be fully sacnned and marked skip after it's first return from
> >
> > Right, but now the 'fast_find_block' is false, and we will call isolation_suitable() to validate the skip flag.
> Right, sorry for missing that.
>
> But it's ok to keep the fast_find_block if you insist and I will just correct the stale

Yes, I still prefer to keep the fast_find_block, since I did not see 
this patch can fix any real issue and might have a side effect for 
fast-find-pageblock(?).

> comment that "fast_find_migrateblock marks a pageblock skipped ..." in next version.

Sure, please do it.