On Tue, May 25, 2021 at 3:06 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > 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. Will update the subject and the commit log accordingly. > > > > > [1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@xxxxxxxxxxxx/ > > > > Reviewed-by: Zi Yan <ziy@xxxxxxxxxx> > > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx> > > This will be required Cc: stable@xxxxxxxxxxxxxxx > (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. Sure. > > > --- > > 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. We should be able to make dump_page() print total mapcount, right? The dump_page() should be just called in some error paths so taking some extra overhead to dump more information seems harmless, or am I missing something? Of course, this can be done in a separate patch. > > I guess leave it as you have it, I don't see an easy better > (without going back to something like the old contortions). > > > } > > > > 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. > > > 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. Aha, yes, good catch. If mapcount is not zero, the refcount freeze won't succeed. > > > 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
