Re: [v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split
From: Hugh Dickins <hughd@google.com>
Date: 2021-05-25 22:06:03
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On Tue, 25 May 2021, Yang Shi wrote:
When debugging the bug reported by Wang Yugui [1], try_to_unmap() may return false positive for PTE-mapped THP since page_mapcount() is used to check if the THP is unmapped, but it just checks compound mapount and head page's mapcount. If the THP is PTE-mapped and head page is not mapped, it may return false positive.
But those false positives did not matter because there was a separate DEBUG_VM check later. It's good to have the link to Wang Yugui's report, but that paragraph is not really about this patch, as it has evolved now: this patch consolidates the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE.
The try_to_unmap() has been changed to void function, so check page_mapped() after it. And changed BUG_ON to WARN_ON since it is not a fatal issue.
The change from DEBUG_VM BUG to VM_WARN_ON_ONCE is the most important part of this, and the reason it's good for stable: and the patch title ought to highlight that, not the page_mapcount business.
[1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/ (local) Reviewed-by: Zi Yan <ziy@nvidia.com> Signed-off-by: Yang Shi <redacted>
This will be required Cc: stable@vger.kernel.org (but we don't want to Cc them on this mail). As I said on the other, I think this should be 1/2 not 2/2.
quoted hunk ↗ jump to hunk
--- v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag since there is no fundamental change against v2. v2: Removed dead code and updated the comment of try_to_unmap() per Zi Yan. mm/huge_memory.c | 17 +++++------------ 1 file changed, 5 insertions(+), 12 deletions(-)diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 80fe642d742d..72d81d8e01b1 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c@@ -2343,6 +2343,8 @@ static void unmap_page(struct page *page) ttu_flags |= TTU_SPLIT_FREEZE; try_to_unmap(page, ttu_flags); + + VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
There is one useful piece of information that dump_page() will not show:
total_mapcount(page). Is there a way of crafting that into the output?
Not with the macros available, I think. Maybe we should be optimistic
and assume I already have the fixes, so not worth trying to refine the
message (but I'm not entirely convinced of that!).
The trouble with
if (VM_WARN_ON_ONCE_PAGE(page_mapped(page), page))
pr_warn("total_mapcount:%d\n", total_mapcount(page));
is that it's printed regardless of the ONCEness. Another "trouble"
is that it's printed so long after the page_mapped(page) check that
it may be 0 by now - but one can see that as itself informative.
I guess leave it as you have it, I don't see an easy better
(without going back to something like the old contortions).
quoted hunk ↗ jump to hunk
} static void remap_page(struct page *page, unsigned int nr)@@ -2653,7 +2655,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) struct deferred_split *ds_queue = get_deferred_split_queue(head); struct anon_vma *anon_vma = NULL; struct address_space *mapping = NULL; - int count, mapcount, extra_pins, ret; + int mapcount, extra_pins, ret;
Remove mapcount too.
quoted hunk ↗ jump to hunk
pgoff_t end; VM_BUG_ON_PAGE(is_huge_zero_page(head), head);@@ -2712,7 +2714,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) } unmap_page(head); - VM_BUG_ON_PAGE(compound_mapcount(head), head); /* block interrupt reentry in xa_lock and spinlock */ local_irq_disable();@@ -2730,7 +2731,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) /* Prevent deferred_split_scan() touching ->_refcount */ spin_lock(&ds_queue->split_queue_lock); - count = page_count(head); mapcount = total_mapcount(head); if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
mapcount was useful for printing in the hand-crafted message deleted, but serves no purpose now: just do the page_ref_freeze() without it.
quoted hunk ↗ jump to hunk
if (!list_empty(page_deferred_list(head))) {@@ -2752,16 +2752,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) __split_huge_page(page, list, end); ret = 0; } else { - if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) { - pr_alert("total_mapcount: %u, page_count(): %u\n", - mapcount, count); - if (PageTail(page)) - dump_page(head, NULL); - dump_page(page, "total_mapcount(head) > 0"); - BUG(); - } spin_unlock(&ds_queue->split_queue_lock); -fail: if (mapping) +fail: + if (mapping) xa_unlock(&mapping->i_pages); local_irq_enable(); remap_page(head, thp_nr_pages(head));-- 2.26.2