On Tue, 24 Mar 2015, Kirill A. Shutemov wrote:
> On Sun, Mar 22, 2015 at 09:40:02PM -0700, Hugh Dickins wrote:
> > (I think Kirill has a problem of that kind in his page_remove_rmap scan).
(And this one I mentioned to you at the conference :)
> >
> > It will be interesting to see what Kirill does to maintain the stats
> > for huge pagecache: but he will have no difficulty in finding fields
> > to store counts, because he's got lots of spare fields in those 511
> > tail pages - that's a useful benefit of the compound page, but does
> > prevent the tails from being used in ordinary ways. (I did try using
> > team_head[1].team_usage for more, but atomicity needs prevented it.)
>
> The patch below should address the race you pointed, if I've got all
> right. Not hugely happy with the change though.
Yes, without thinking too hard about it, something like what you have
below should do it. Not very pretty; maybe a neater idea will come up.
Hugh
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 435c90f59227..a3e6b35520f8 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -423,8 +423,17 @@ static inline void page_mapcount_reset(struct page *page)
>
> static inline int page_mapcount(struct page *page)
> {
> + int ret;
> VM_BUG_ON_PAGE(PageSlab(page), page);
> - return atomic_read(&page->_mapcount) + compound_mapcount(page) + 1;
> + ret = atomic_read(&page->_mapcount) + 1;
> + if (compound_mapcount(page)) {
> + /*
> + * positive compound_mapcount() offsets ->_mapcount by one --
> + * substract here.
> + */
> + ret += compound_mapcount(page) - 1;
> + }
> + return ret;
> }
>
> static inline int page_count(struct page *page)
> diff --git a/mm/rmap.c b/mm/rmap.c
> index fc6eee4ed476..f4ab976276e7 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1066,9 +1066,17 @@ void do_page_add_anon_rmap(struct page *page,
> * disabled.
> */
> if (compound) {
> + int i;
> VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> __inc_zone_page_state(page,
> NR_ANON_TRANSPARENT_HUGEPAGES);
> + /*
> + * While compound_mapcount() is positive we keep *one*
> + * mapcount reference in all subpages. It's required
> + * for atomic removal from rmap.
> + */
> + for (i = 0; i < nr; i++)
> + atomic_set(&page[i]._mapcount, 0);
> }
> __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, nr);
> }
> @@ -1103,10 +1111,19 @@ void page_add_new_anon_rmap(struct page *page,
> VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
> SetPageSwapBacked(page);
> if (compound) {
> + int i;
> +
> VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> /* increment count (starts at -1) */
> atomic_set(compound_mapcount_ptr(page), 0);
> __inc_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> + /*
> + * While compound_mapcount() is positive we keep *one* mapcount
> + * reference in all subpages. It's required for atomic removal
> + * from rmap.
> + */
> + for (i = 0; i < nr; i++)
> + atomic_set(&page[i]._mapcount, 0);
> } else {
> /* Anon THP always mapped first with PMD */
> VM_BUG_ON_PAGE(PageTransCompound(page), page);
> @@ -1174,9 +1191,6 @@ out:
> */
> void page_remove_rmap(struct page *page, bool compound)
> {
> - int nr = compound ? hpage_nr_pages(page) : 1;
> - bool partial_thp_unmap;
> -
> if (!PageAnon(page)) {
> VM_BUG_ON_PAGE(compound && !PageHuge(page), page);
> page_remove_file_rmap(page);
> @@ -1184,10 +1198,20 @@ void page_remove_rmap(struct page *page, bool compound)
> }
>
> /* page still mapped by someone else? */
> - if (!atomic_add_negative(-1, compound ?
> - compound_mapcount_ptr(page) :
> - &page->_mapcount))
> + if (compound) {
> + int i;
> +
> + VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> + if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
> + return;
> + __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> + for (i = 0; i < hpage_nr_pages(page); i++)
> + page_remove_rmap(page + i, false);
> return;
> + } else {
> + if (!atomic_add_negative(-1, &page->_mapcount))
> + return;
> + }
>
> /* Hugepages are not counted in NR_ANON_PAGES for now. */
> if (unlikely(PageHuge(page)))
> @@ -1198,26 +1222,12 @@ void page_remove_rmap(struct page *page, bool compound)
> * these counters are not modified in interrupt context, and
> * pte lock(a spinlock) is held, which implies preemption disabled.
> */
> - if (compound) {
> - int i;
> - VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> - __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> - /* The page can be mapped with ptes */
> - for (i = 0; i < hpage_nr_pages(page); i++)
> - if (page_mapcount(page + i))
> - nr--;
> - partial_thp_unmap = nr != hpage_nr_pages(page);
> - } else if (PageTransCompound(page)) {
> - partial_thp_unmap = !compound_mapcount(page);
> - } else
> - partial_thp_unmap = false;
> -
> - __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, -nr);
> + __dec_zone_page_state(page, NR_ANON_PAGES);
>
> if (unlikely(PageMlocked(page)))
> clear_page_mlock(page);
>
> - if (partial_thp_unmap)
> + if (PageTransCompound(page))
> deferred_split_huge_page(compound_head(page));
>
> /*
> --
> Kirill A. Shutemov
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