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

[Baolin Wang]

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.

> 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.

> fast_find_migrateblock and it should be skipped.
> Thanks!