On Thu, 13 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. > > Use total_mapcount() instead of page_mapcount() for try_to_unmap() and > do so for the VM_BUG_ON_PAGE in split_huge_page_to_list as well. > > This changed the semantic of try_to_unmap(), but I don't see there is > any usecase that expects try_to_unmap() just unmap one subpage of a huge > page. So using page_mapcount() seems like a bug. > > [1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@xxxxxxxxxxxx/ > > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx> I don't object to this patch, I've no reason to NAK it; but I'll point out a few deficiencies which might make you want to revisit it. > --- > v2: Removed dead code and updated the comment of try_to_unmap() per Zi > Yan. > > mm/huge_memory.c | 11 +---------- > mm/rmap.c | 10 ++++++---- > 2 files changed, 7 insertions(+), 14 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 63ed6b25deaa..3b08b9ba1578 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -2348,7 +2348,6 @@ static void unmap_page(struct page *page) > ttu_flags |= TTU_SPLIT_FREEZE; > > unmap_success = try_to_unmap(page, ttu_flags); > - VM_BUG_ON_PAGE(!unmap_success, page); The unused variable unmap_success has already been reported and dealt with. But I couldn't tell what you intended: why change try_to_unmap()'s output, if you then ignore it? > } > > static void remap_page(struct page *page, unsigned int nr) > @@ -2718,7 +2717,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) > } > > unmap_page(head); > - VM_BUG_ON_PAGE(compound_mapcount(head), head); > + VM_BUG_ON_PAGE(total_mapcount(head), head); And having forced try_to_unmap() to do the expensive-on-a-THP total_mapcount() calculation, you now repeat it here. Better to stick with the previous VM_BUG_ON_PAGE(!unmap_success). Or better a VM_WARN_ONCE(), accompanied by dump_page()s as before, to get some perhaps useful info out, which this patch has deleted. Probably better inside unmap_page() than cluttering up here. VM_WARN_ONCE() because nothing in this patch fixes whatever Wang Yugui is suffering from; and (aside from the BUG()) it's harmless, because there are other ways in which the page_ref_freeze() can fail, and that is allowed for. We would like to know when this problem occurs: there is something wrong, but no reason to crash. > > /* block interrupt reentry in xa_lock and spinlock */ > local_irq_disable(); > @@ -2758,14 +2757,6 @@ 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(); > - } This has always looked ugly (as if Kirill had hit an unsolved case), so it is nice to remove it; but you're losing the dump_page() info, and not really gaining anything more than a cosmetic cleanup. > spin_unlock(&ds_queue->split_queue_lock); > fail: if (mapping) > xa_unlock(&mapping->i_pages); > diff --git a/mm/rmap.c b/mm/rmap.c > index 693a610e181d..f52825b1330d 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1742,12 +1742,14 @@ static int page_not_mapped(struct page *page) > } > > /** > - * try_to_unmap - try to remove all page table mappings to a page > - * @page: the page to get unmapped > + * try_to_unmap - try to remove all page table mappings to a page and the > + * compound page it belongs to > + * @page: the page or the subpages of compound page to get unmapped > * @flags: action and flags > * > * Tries to remove all the page table entries which are mapping this > - * page, used in the pageout path. Caller must hold the page lock. > + * page and the compound page it belongs to, used in the pageout path. > + * Caller must hold the page lock. > * > * If unmap is successful, return true. Otherwise, false. > */ > @@ -1777,7 +1779,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags) > else > rmap_walk(page, &rwc); > > - return !page_mapcount(page) ? true : false; > + return !total_mapcount(page) ? true : false; That always made me wince: "return !total_mapcount(page);" surely. Or slightly better, "return !page_mapped(page);", since at least that one breaks out as soon as it sees a mapcount. Though I guess I'm being silly there, since that case should never occur, so both total_mapcount() and page_mapped() scan through all pages. Or better, change try_to_unmap() to void: most callers ignore its return value anyway, and make their own decisions; the remaining few could be changed to do the same. Though again, I may be being silly, since the expensive THP case is not the common case. > } > > /** > -- > 2.26.2
