| Message ID | 20210123154320.24278-1-vvghjk1234@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm/compactoin: Fix edge case of fast_find_migrateblock() | expand |
On 1/23/21 4:43 PM, Wonhyuk Yang wrote: > In the fast_find_migrateblock(), It iterate freelist to find > proper pageblock. But there are two edge cases. First, if the page > we found is equal to cc->migrate_pfn, it is considered that we > didn't found suitable pageblock. Second, if the loop was terminated > because order is less than PAGE_ALLOC_COSTLY_ORDER, it could > be considered that we found suitable one. > > Fixes: 70b44595eafe9 ("mm, compaction: use free lists to quickly locate > a migration source") > > Signed-off-by: Wonhyuk Yang <vvghjk1234@gmail.com> Seems correct, although it's quite subtle code and it's been a long time... Acked-by: Vlastimil Babka <vbabka@suse.cz> Maybe instead of replacing one magic value of 'pfn' with another (cc->migrate_pfn with high_pfn) I would just add a "bool found" and use it appropriately. Would make the function less subtle perhaps. While reviewing I found more potentially questionable parts, if you're interested: - if we go through "if (get_pageblock_skip(freepage))... continue;" we are increasing nr_scanned++; but not checking the limit. - the "if (list_is_last(freelist, &freepage->lru)) break;" part seems unneccesary? if we are on the last page and just "continue;" the list_for_each_entry() iteration should stop anyway. > --- > mm/compaction.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/mm/compaction.c b/mm/compaction.c > index e5acb9714436..46f49e6b7d1a 100644 > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -1742,8 +1742,8 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) > distance >>= 2; > high_pfn = pageblock_start_pfn(cc->migrate_pfn + distance); > > - for (order = cc->order - 1; > - order >= PAGE_ALLOC_COSTLY_ORDER && pfn == cc->migrate_pfn && nr_scanned < limit; > + for (order = cc->order - 1, pfn = high_pfn; > + order >= PAGE_ALLOC_COSTLY_ORDER && pfn == high_pfn && nr_scanned < limit; > order--) { > struct free_area *area = &cc->zone->free_area[order]; > struct list_head *freelist; > @@ -1785,7 +1785,6 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) > } > > if (nr_scanned >= limit) { > - cc->fast_search_fail++; > move_freelist_tail(freelist, freepage); > break; > } > @@ -1799,9 +1798,10 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) > * If fast scanning failed then use a cached entry for a page block > * that had free pages as the basis for starting a linear scan. > */ > - if (pfn == cc->migrate_pfn) > + if (pfn == high_pfn) { > + cc->fast_search_fail++; > pfn = reinit_migrate_pfn(cc); > - > + } > return pfn; > } > >
On Tue, Jan 26, 2021 at 1:49 AM Vlastimil Babka <vbabka@suse.cz> wrote: > > Maybe instead of replacing one magic value of 'pfn' with another > (cc->migrate_pfn with high_pfn) I would just add a "bool found" and use it > appropriately. Would make the function less subtle perhaps. I agree. Using that bool variable will look good. > > While reviewing I found more potentially questionable parts, if you're interested: > > - if we go through "if (get_pageblock_skip(freepage))... continue;" we are > increasing nr_scanned++; but not checking the limit. Yes, you've got a point. I think nr_scanned checking should be first. > > - the "if (list_is_last(freelist, &freepage->lru)) break;" part seems > unneccesary? if we are on the last page and just "continue;" the > list_for_each_entry() iteration should stop anyway. > Yes It seems unnecessary. only if the skipped block was reordered, it would be required. Then I will reflect your opinion and send a new version.
diff --git a/mm/compaction.c b/mm/compaction.c index e5acb9714436..46f49e6b7d1a 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -1742,8 +1742,8 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) distance >>= 2; high_pfn = pageblock_start_pfn(cc->migrate_pfn + distance); - for (order = cc->order - 1; - order >= PAGE_ALLOC_COSTLY_ORDER && pfn == cc->migrate_pfn && nr_scanned < limit; + for (order = cc->order - 1, pfn = high_pfn; + order >= PAGE_ALLOC_COSTLY_ORDER && pfn == high_pfn && nr_scanned < limit; order--) { struct free_area *area = &cc->zone->free_area[order]; struct list_head *freelist; @@ -1785,7 +1785,6 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) } if (nr_scanned >= limit) { - cc->fast_search_fail++; move_freelist_tail(freelist, freepage); break; } @@ -1799,9 +1798,10 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) * If fast scanning failed then use a cached entry for a page block * that had free pages as the basis for starting a linear scan. */ - if (pfn == cc->migrate_pfn) + if (pfn == high_pfn) { + cc->fast_search_fail++; pfn = reinit_migrate_pfn(cc); - + } return pfn; }
In the fast_find_migrateblock(), It iterate freelist to find proper pageblock. But there are two edge cases. First, if the page we found is equal to cc->migrate_pfn, it is considered that we didn't found suitable pageblock. Second, if the loop was terminated because order is less than PAGE_ALLOC_COSTLY_ORDER, it could be considered that we found suitable one. Fixes: 70b44595eafe9 ("mm, compaction: use free lists to quickly locate a migration source") Signed-off-by: Wonhyuk Yang <vvghjk1234@gmail.com> --- mm/compaction.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)