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@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. > --- > 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). > } > > 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. > 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
