| Message ID | 20120925143931.f404ca22.akpm@linux-foundation.org |
|---|---|
| State | New |
| Headers | show |
On Tue, Sep 25, 2012 at 02:39:31PM -0700, Andrew Morton wrote: > On Tue, 25 Sep 2012 17:13:27 +0900 > Minchan Kim <minchan@kernel.org> wrote: > > > I see. To me, your saying is better than current comment. > > I hope comment could be more explicit. > > > > diff --git a/mm/compaction.c b/mm/compaction.c > > index df01b4e..f1d2cc7 100644 > > --- a/mm/compaction.c > > +++ b/mm/compaction.c > > @@ -542,8 +542,9 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > > * splitting and collapsing (collapsing has already happened > > * if PageLRU is set) but the lock is not necessarily taken > > * here and it is wasteful to take it just to check transhuge. > > - * Check transhuge without lock and skip if it's either a > > - * transhuge or hugetlbfs page. > > + * Check transhuge without lock and *skip* if it's either a > > + * transhuge or hugetlbfs page because it's not safe to call > > + * compound_order. > > */ > > if (PageTransHuge(page)) { > > if (!locked) > > Going a bit further: > > --- a/mm/compaction.c~mm-compaction-acquire-the-zone-lru_lock-as-late-as-possible-fix > +++ a/mm/compaction.c > @@ -415,7 +415,8 @@ isolate_migratepages_range(struct zone * > * if PageLRU is set) but the lock is not necessarily taken > * here and it is wasteful to take it just to check transhuge. > * Check transhuge without lock and skip if it's either a > - * transhuge or hugetlbfs page. > + * transhuge or hugetlbfs page because calling compound_order() > + * requires lru_lock to exclude isolation and splitting. > */ > if (PageTransHuge(page)) { > if (!locked) > _ > > > but... the requirement to hold lru_lock for compound_order() is news > to me. It doesn't seem to be written down or explained anywhere, and > one wonders why the cheerily undocumented compound_lock() doesn't have > this effect. What's going on here?? First of all, I don't know why we should mention hugetlbfs in comment. I don't know hugetlbfs well so I had a time to look through code but can't find a place setting PG_lru so I'm not sure hugetlbfs page can reach on this code. Please correct me if I was wrong. On THP, I think compound_lock you mentioned is okay, too but I think it's sort of optimization because we don't need both lru_lock and compound_lock. If we hold lru_lock, we can't prevent race with __split_huge_page_refcount so that the page couldn't be freed. Namely, it's safe to call compound_order. > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
On Tue, Sep 25, 2012 at 02:39:31PM -0700, Andrew Morton wrote: > On Tue, 25 Sep 2012 17:13:27 +0900 > Minchan Kim <minchan@kernel.org> wrote: > > > I see. To me, your saying is better than current comment. > > I hope comment could be more explicit. > > > > diff --git a/mm/compaction.c b/mm/compaction.c > > index df01b4e..f1d2cc7 100644 > > --- a/mm/compaction.c > > +++ b/mm/compaction.c > > @@ -542,8 +542,9 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > > * splitting and collapsing (collapsing has already happened > > * if PageLRU is set) but the lock is not necessarily taken > > * here and it is wasteful to take it just to check transhuge. > > - * Check transhuge without lock and skip if it's either a > > - * transhuge or hugetlbfs page. > > + * Check transhuge without lock and *skip* if it's either a > > + * transhuge or hugetlbfs page because it's not safe to call > > + * compound_order. > > */ > > if (PageTransHuge(page)) { > > if (!locked) > > Going a bit further: > > --- a/mm/compaction.c~mm-compaction-acquire-the-zone-lru_lock-as-late-as-possible-fix > +++ a/mm/compaction.c > @@ -415,7 +415,8 @@ isolate_migratepages_range(struct zone * > * if PageLRU is set) but the lock is not necessarily taken > * here and it is wasteful to take it just to check transhuge. > * Check transhuge without lock and skip if it's either a > - * transhuge or hugetlbfs page. > + * transhuge or hugetlbfs page because calling compound_order() > + * requires lru_lock to exclude isolation and splitting. > */ > if (PageTransHuge(page)) { > if (!locked) > _ > > but... the requirement to hold lru_lock for compound_order() is news > to me. It doesn't seem to be written down or explained anywhere, and > one wonders why the cheerily undocumented compound_lock() doesn't have > this effect. What's going on here?? > The lru_lock is not *required* for compound_order(). Normally, users of compound_order() know that the page is not going to collapse underneath them. The slub allocator is not going to have a compound page it controls disappear unexpectedly and does not need additional locking for example. In the case where we are potentially dealing with a THP page, we have to take into account if it can collapse underneath us. In this case, there is a race between when when PageTransHuge is checked and compound_order is called. The race is probably harmless but it was easy to take into account in this case. I think the comment saying that lru_lock is required is misleading. How about this? * Check TransHuge without lock and skip the whole * pageblock if it's either a transhuge or hugetlbfs page * as calling compound_order without preventing THP * splitting the page underneath us may return * surprising results.
--- a/mm/compaction.c~mm-compaction-acquire-the-zone-lru_lock-as-late-as-possible-fix +++ a/mm/compaction.c @@ -415,7 +415,8 @@ isolate_migratepages_range(struct zone * * if PageLRU is set) but the lock is not necessarily taken * here and it is wasteful to take it just to check transhuge. * Check transhuge without lock and skip if it's either a - * transhuge or hugetlbfs page. + * transhuge or hugetlbfs page because calling compound_order() + * requires lru_lock to exclude isolation and splitting. */ if (PageTransHuge(page)) { if (!locked)