On Tue, May 25, 2021 at 4:58 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > On Tue, 25 May 2021, Yang Shi wrote: > > 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. > > Thanks! > > > > > > > > > > > > > > [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. > > Great. > > > > > > > > > > --- > > > > 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 didn't want to ask that of you, but yes, if you're willing to add > total_mapcount() into dump_page(), I think that would be ideal; and > could be helpful for other cases too. > > Looking through total_mapcount(), I think it's safe to call from > dump_page() - I always worry about extending crash info with > something that depends on a maybe-corrupted pointer which would > generate a further crash and either recurse or truncate the output - > but please check that carefully. Yes, it is possible. If the THP is being split, some VM_BUG_* might be triggered if total_mapcount() is called. But it is still feasible to print total mapcount as long as we implement a more robust version for dump_page(). > > Yes, a separate patch please: which can come later on, and no > need for stable for that one, but good to know it's coming. > > Thanks, > Hugh
