On Tue, May 24, 2022 at 08:43:27AM -0700, Minchan Kim wrote: > On Tue, May 24, 2022 at 11:19:37AM -0300, Jason Gunthorpe wrote: > > On Mon, May 23, 2022 at 10:16:58PM -0700, Minchan Kim wrote: > > > On Mon, May 23, 2022 at 07:55:25PM -0700, John Hubbard wrote: > > > > On 5/23/22 09:33, Minchan Kim wrote: > > > > ... > > > > > > So then: > > > > > > > > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > > > > > index 0e42038382c1..b404f87e2682 100644 > > > > > > +++ b/mm/page_alloc.c > > > > > > @@ -482,7 +482,12 @@ unsigned long __get_pfnblock_flags_mask(const struct page *page, > > > > > > word_bitidx = bitidx / BITS_PER_LONG; > > > > > > bitidx &= (BITS_PER_LONG-1); > > > > > > > > > > > > - word = bitmap[word_bitidx]; > > > > > > + /* > > > > > > + * This races, without locks, with set_pageblock_migratetype(). Ensure > > > > > set_pfnblock_flags_mask would be better? > > > > > > + * a consistent (non-tearing) read of the memory array, so that results, > > > > > > > > > > Thanks for proceeding and suggestion, John. > > > > > > > > > > IIUC, the load tearing wouldn't be an issue since [1] fixed the issue. > > > > > > > > Did it? [1] fixed something, but I'm not sure we can claim that that > > > > code is now safe against tearing in all possible cases, especially given > > > > the recent discussion here. Specifically, having this code do a read, > > > > then follow that up with calculations, seems correct. Anything else is > > > > > > The load tearing you are trying to explain in the comment would be > > > solved by [1] since the bits will always align on a word and accessing > > > word size based on word aligned address is always atomic so there is > > > no load tearing problem IIUC. > > > > That is not technically true. It is exactly the sort of thing > > READ_ONCE is intended to guard against. > > Oh, does word access based on the aligned address still happen > load tearing? > > I just referred to > https://elixir.bootlin.com/linux/latest/source/Documentation/memory-barriers.txt#L1759 I read that as saying load tearing is technically allowed but doesn't happen in gcc, and so must use the _ONCE macros. > I didn't say it doesn't refetch the value without the READ_ONCE. > > What I am saying is READ_ONCE(bitmap_word_bitidx] prevents "refetching" > issue rather than "tearing" issue in specific __get_pfnblock_flags_mask > context because I though there is no load-tearing issue there since > bitmap is word-aligned/accessed. No? It does both. AFAIK our memory model has no guarentees on what naked C statements will do. Tearing, multi-load, etc - it is all technically permitted. Use the proper accessors. Jason
