On 13.04.22 10:25, Vlastimil Babka wrote: > On 3/29/22 18:04, David Hildenbrand wrote: >> The basic question we would like to have a reliable and efficient answer >> to is: is this anonymous page exclusive to a single process or might it >> be shared? We need that information for ordinary/single pages, hugetlb >> pages, and possibly each subpage of a THP. >> >> Introduce a way to mark an anonymous page as exclusive, with the >> ultimate goal of teaching our COW logic to not do "wrong COWs", whereby >> GUP pins lose consistency with the pages mapped into the page table, >> resulting in reported memory corruptions. >> >> Most pageflags already have semantics for anonymous pages, however, >> PG_mappedtodisk should never apply to pages in the swapcache, so let's >> reuse that flag. >> >> As PG_has_hwpoisoned also uses that flag on the second tail page of a >> compound page, convert it to PG_error instead, which is marked as >> PF_NO_TAIL, so never used for tail pages. >> >> Use custom page flag modification functions such that we can do >> additional sanity checks. The semantics we'll put into some kernel doc >> in the future are: >> >> " >> PG_anon_exclusive is *usually* only expressive in combination with a >> page table entry. Depending on the page table entry type it might >> store the following information: >> >> Is what's mapped via this page table entry exclusive to the >> single process and can be mapped writable without further >> checks? If not, it might be shared and we might have to COW. >> >> For now, we only expect PTE-mapped THPs to make use of >> PG_anon_exclusive in subpages. For other anonymous compound >> folios (i.e., hugetlb), only the head page is logically mapped and >> holds this information. >> >> For example, an exclusive, PMD-mapped THP only has PG_anon_exclusive >> set on the head page. When replacing the PMD by a page table full >> of PTEs, PG_anon_exclusive, if set on the head page, will be set on >> all tail pages accordingly. Note that converting from a PTE-mapping >> to a PMD mapping using the same compound page is currently not >> possible and consequently doesn't require care. >> >> If GUP wants to take a reliable pin (FOLL_PIN) on an anonymous page, >> it should only pin if the relevant PG_anon_bit is set. In that case, > > ^ PG_anon_exclusive bit ? > >> the pin will be fully reliable and stay consistent with the pages >> mapped into the page table, as the bit cannot get cleared (e.g., by >> fork(), KSM) while the page is pinned. For anonymous pages that >> are mapped R/W, PG_anon_exclusive can be assumed to always be set >> because such pages cannot possibly be shared. >> >> The page table lock protecting the page table entry is the primary >> synchronization mechanism for PG_anon_exclusive; GUP-fast that does >> not take the PT lock needs special care when trying to clear the >> flag. >> >> Page table entry types and PG_anon_exclusive: >> * Present: PG_anon_exclusive applies. >> * Swap: the information is lost. PG_anon_exclusive was cleared. >> * Migration: the entry holds this information instead. >> PG_anon_exclusive was cleared. >> * Device private: PG_anon_exclusive applies. >> * Device exclusive: PG_anon_exclusive applies. >> * HW Poison: PG_anon_exclusive is stale and not changed. >> >> If the page may be pinned (FOLL_PIN), clearing PG_anon_exclusive is >> not allowed and the flag will stick around until the page is freed >> and folio->mapping is cleared. > > Or also if it's unpinned? I'm afraid I didn't get your question. Once the page is no longer pinned, we can succeed in clearing PG_anon_exclusive (just like pinning never happened). Does that answer your question? > >> " >> >> We won't be clearing PG_anon_exclusive on destructive unmapping (i.e., >> zapping) of page table entries, page freeing code will handle that when >> also invalidate page->mapping to not indicate PageAnon() anymore. >> Letting information about exclusivity stick around will be an important >> property when adding sanity checks to unpinning code. >> >> Note that we properly clear the flag in free_pages_prepare() via >> PAGE_FLAGS_CHECK_AT_PREP for each individual subpage of a compound page, >> so there is no need to manually clear the flag. >> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Thanks! > >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -3663,6 +3663,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> goto out_nomap; >> } >> >> + /* >> + * PG_anon_exclusive reuses PG_mappedtodisk for anon pages. A swap pte >> + * must never point at an anonymous page in the swapcache that is >> + * PG_anon_exclusive. Sanity check that this holds and especially, that >> + * no filesystem set PG_mappedtodisk on a page in the swapcache. Sanity >> + * check after taking the PT lock and making sure that nobody >> + * concurrently faulted in this page and set PG_anon_exclusive. >> + */ >> + BUG_ON(!PageAnon(page) && PageMappedToDisk(page)); >> + BUG_ON(PageAnon(page) && PageAnonExclusive(page)); >> + > > Hmm, dunno why not VM_BUG_ON? Getting PageAnonExclusive accidentally set by a file system would result in an extremely unpleasant security issue. I most surely want to catch something like that in any case, especially in the foreseeable future. -- Thanks, David / dhildenb
