| Message ID | 20250225-clarify-steal-v3-2-f2550ead0139@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/page_alloc: Some clarifications for migratetype fallback | expand |
On 2/25/25 4:29 PM, Brendan Jackman wrote: > There's lots of text here but it's a little hard to follow, this is an > attempt to break it up and align its structure more closely with the > code. > > Reword the top-level function comment to just explain what question the > function answers from the point of view of the caller. > > Break up the internal logic into different sections that can have their > own commentary describing why that part of the rationale is present. > > Note the page_groupy_by_mobility_disabled logic is not explained in the grouping > commentary, that is outside the scope of this patch... > > Signed-off-by: Brendan Jackman <jackmanb@google.com> Johannes suggested moving the checks to the caller and removing this function but with this kind of detailed commentary I guess it's better to keep it as a separate function. > --- > mm/page_alloc.c | 39 +++++++++++++++++++++++---------------- > 1 file changed, 23 insertions(+), 16 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 5e694046ef92965b34d4831e96d92f02681a8b45..475ec1284033acec69da4a39dd4e7d7fbaee6d0f 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1941,16 +1941,9 @@ static inline bool boost_watermark(struct zone *zone) > } > > /* > - * When we are falling back to another migratetype during allocation, try to > - * claim entire blocks to satisfy further allocations, instead of polluting > - * multiple pageblocks. > - * > - * If we are stealing a relatively large buddy page, it is likely there will be > - * more free pages in the pageblock, so try to claim the whole block. For > - * reclaimable and unmovable allocations, we claim the whole block regardless of > - * page size, as fragmentation caused by those allocations polluting movable > - * pageblocks is worse than movable allocations stealing from unmovable and > - * reclaimable pageblocks. > + * When we are falling back to another migratetype during allocation, should we > + * try to claim an entire block to satisfy further allocations, instead of > + * polluting multiple pageblocks? > */ > static bool should_claim_block(unsigned int order, int start_mt) > { > @@ -1964,6 +1957,26 @@ static bool should_claim_block(unsigned int order, int start_mt) > if (order >= pageblock_order) > return true; > > + /* > + * Above a certain threshold, always try to claim, as it's likely there > + * will be more free pages in the pageblock. > + */ > + if (order >= pageblock_order / 2) > + return true; > + > + /* > + * Unmovable/reclaimable allocations would cause permanent > + * fragmentations if they fell back to allocating from a movable block > + * (polluting it), so we try to claim the whole block regardless of the > + * allocation size. Later movable allocations can always steal from this > + * block, which is less problematic. > + */ > + if (start_mt == MIGRATE_RECLAIMABLE || start_mt == MIGRATE_UNMOVABLE) > + return true; > + > + if (page_group_by_mobility_disabled) > + return true; > + > /* > * Movable pages won't cause permanent fragmentation, so when you alloc > * small pages, you just need to temporarily steal unmovable or > @@ -1972,12 +1985,6 @@ static bool should_claim_block(unsigned int order, int start_mt) > * and the next movable allocation may not need to steal. Unmovable and > * reclaimable allocations need to actually claim the whole block. > */ This block could be also massaged? I'd unify the style so it's "we" everywhere and not suddenly "you". The last sentence is redundant with the comment you added above reclaimable/unmovable. The text should be just about movable allocations in the case of small enough order. > - if (order >= pageblock_order / 2 || > - start_mt == MIGRATE_RECLAIMABLE || > - start_mt == MIGRATE_UNMOVABLE || > - page_group_by_mobility_disabled) > - return true; > - > return false; > } > >
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 5e694046ef92965b34d4831e96d92f02681a8b45..475ec1284033acec69da4a39dd4e7d7fbaee6d0f 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1941,16 +1941,9 @@ static inline bool boost_watermark(struct zone *zone) } /* - * When we are falling back to another migratetype during allocation, try to - * claim entire blocks to satisfy further allocations, instead of polluting - * multiple pageblocks. - * - * If we are stealing a relatively large buddy page, it is likely there will be - * more free pages in the pageblock, so try to claim the whole block. For - * reclaimable and unmovable allocations, we claim the whole block regardless of - * page size, as fragmentation caused by those allocations polluting movable - * pageblocks is worse than movable allocations stealing from unmovable and - * reclaimable pageblocks. + * When we are falling back to another migratetype during allocation, should we + * try to claim an entire block to satisfy further allocations, instead of + * polluting multiple pageblocks? */ static bool should_claim_block(unsigned int order, int start_mt) { @@ -1964,6 +1957,26 @@ static bool should_claim_block(unsigned int order, int start_mt) if (order >= pageblock_order) return true; + /* + * Above a certain threshold, always try to claim, as it's likely there + * will be more free pages in the pageblock. + */ + if (order >= pageblock_order / 2) + return true; + + /* + * Unmovable/reclaimable allocations would cause permanent + * fragmentations if they fell back to allocating from a movable block + * (polluting it), so we try to claim the whole block regardless of the + * allocation size. Later movable allocations can always steal from this + * block, which is less problematic. + */ + if (start_mt == MIGRATE_RECLAIMABLE || start_mt == MIGRATE_UNMOVABLE) + return true; + + if (page_group_by_mobility_disabled) + return true; + /* * Movable pages won't cause permanent fragmentation, so when you alloc * small pages, you just need to temporarily steal unmovable or @@ -1972,12 +1985,6 @@ static bool should_claim_block(unsigned int order, int start_mt) * and the next movable allocation may not need to steal. Unmovable and * reclaimable allocations need to actually claim the whole block. */ - if (order >= pageblock_order / 2 || - start_mt == MIGRATE_RECLAIMABLE || - start_mt == MIGRATE_UNMOVABLE || - page_group_by_mobility_disabled) - return true; - return false; }
There's lots of text here but it's a little hard to follow, this is an attempt to break it up and align its structure more closely with the code. Reword the top-level function comment to just explain what question the function answers from the point of view of the caller. Break up the internal logic into different sections that can have their own commentary describing why that part of the rationale is present. Note the page_groupy_by_mobility_disabled logic is not explained in the commentary, that is outside the scope of this patch... Signed-off-by: Brendan Jackman <jackmanb@google.com> --- mm/page_alloc.c | 39 +++++++++++++++++++++++---------------- 1 file changed, 23 insertions(+), 16 deletions(-)