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 5ruf4iaN 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6 Feb 2026, at 18:06, Zi Yan wrote: > On 6 Feb 2026, at 17:37, Mikhail Gavrilov wrote: > >> On Sat, Feb 7, 2026 at 3:16=E2=80=AFAM Mikhail Gavrilov >> wrote: >>> >>> Hi Zi, >>> Thanks for the deep investigation! >>> So the actual culprit is KASAN's kasan_save_stack() leaving non-zero >>> page->private. >>> That explains why it only reproduces with KASAN enabled. >>> Looking at the code, kasan_save_stack() doesn't seem to use >>> page->private directly - it goes through stack_depot. Is stack_depot >>> the actual culprit? >>> Happy to help investigate further if needed. >>> Regarding the fix location - even if we fix KASAN/stack_depot, >>> split_page() clearing page->private still seems like the right >>> defensive fix. >>> The contract for split_page() is that it produces independent usable >>> pages, and page->private being clean is part of that. >>> Other code could potentially leave stale values too. >>> I can share my .config if still needed, but it sounds like you've >>> already reproduced it. >>> >> >> I think I found it. Looking at mm/internal.h:811, prep_compound_tail() >> clears page->private for tail pages, >> but it's only called for compound pages (__GFP_COMP). >> Before commit 3b8000ae185c, vmalloc used __GFP_COMP, so tail pages got >> their page->private cleared via prep_compound_tail(). >> After that commit dropped __GFP_COMP, tail pages keep stale values >> from buddy allocator (which uses page->private for order). >> So the stale value comes from buddy allocator's set_buddy_order() at >> mm/page_alloc.c:755, >> and __del_page_from_free_list() at line 898 only clears the head page's = private. > > set_buddy_order() also only set head page=E2=80=99s private. And at each = buddy > page merge, any buddy found in free list gets its head page=E2=80=99s pri= vate > cleared in __del_page_from_free_list(). The final merged free page > gets its private set by set_buddy_order() at done_merging. There should > not be any stale values in any page=E2=80=99s private, if I read the code= correctly. > > If it is the problem of buddy allocator leaving stale private values, > the problem would be reproducible with and without KASAN. > OK, it seems that both slub and shmem do not reset ->private when freeing pages/folios. And tail page's private is not zero, because when a page with non zero private is freed and gets merged with a lower buddy, its private is not set to 0 in the code path. The patch below seems to fix the issue, since I am at Iteration 104 and cou= nting. I also put a VM_BUG_ON(page->private) in free_pages_prepare() and it is not triggered either. diff --git a/mm/shmem.c b/mm/shmem.c index ec6c01378e9d..546e193ef993 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2437,8 +2437,10 @@ static int shmem_swapin_folio(struct inode *inode, p= goff_t index, failed_nolock: if (skip_swapcache) swapcache_clear(si, folio->swap, folio_nr_pages(folio)); - if (folio) + if (folio) { + folio->swap.val =3D 0; folio_put(folio); + } put_swap_device(si); return error; diff --git a/mm/slub.c b/mm/slub.c index f77b7407c51b..2cdab6d66e1a 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -3311,6 +3311,7 @@ static void __free_slab(struct kmem_cache *s, struct = slab *slab) __slab_clear_pfmemalloc(slab); page->mapping =3D NULL; + page->private =3D 0; __ClearPageSlab(page); mm_account_reclaimed_pages(pages); unaccount_slab(slab, order, s); But I am not sure if that is all. Maybe the patch below on top is needed to= find all violators and still keep the system running. I also would like to hear from others on= whether page->private should be reset or not before free_pages_prepare(). diff --git a/mm/page_alloc.c b/mm/page_alloc.c index cbf758e27aa2..9058f94b0667 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1430,6 +1430,8 @@ __always_inline bool free_pages_prepare(struct page *= page, page_cpupid_reset_last(page); page->flags.f &=3D ~PAGE_FLAGS_CHECK_AT_PREP; + VM_WARN_ON_ONCE(page->private); + page->private =3D 0; reset_page_owner(page, order); page_table_check_free(page, order); pgalloc_tag_sub(page, 1 << order); -- Best Regards, Yan, Zi