From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 01FA848662F; Wed, 19 Aug 2026 16:09:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787155761; cv=none; b=WbJx8TESXXv1oVntN0iONohgzT6cOjbZKMOV1LbzWZpnF6t0B27zmX5FPDK4DSJ1ccg8GRb6pX+5U0IYb1r2iuLt83mqm92FQp8QpYeTzc+7PL3AixDeqMxMlOUy1jQO29hoX2dvLsX2i329RUpRmxfVQPAOduZ04Mv2zK7e+xo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787155761; c=relaxed/simple; bh=vyBInu7Tm9q5ns7vgEaEBClsZMsrSMd4w13hBn9wiMY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=WVWp+wcct/4ZiJqYYWyPgM9iSbFYw6hlEC/2BH1fmdqcXjay8YKT1Y0li08j1IWBY/yo7YwUCMNLRtBdu6UV4sQwhTEzlNoXhOJfZHst4KIrMNPqiJLERahXVSYiL2541TWkri4S/RnA1lo2SaRAgwr1dNqnidhad+o2b/6MJds= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HOJKhE6Q; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="HOJKhE6Q" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 215971F000E9; Wed, 19 Aug 2026 16:09:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787155759; bh=63V/Ij2B9WrOrIN2e2zHaSp/qasYhzA6FfRA6dqG5dE=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=HOJKhE6QS17uYsGjRHWX/10MHY1v1FQtfamHsWROPjC4W1e30OS75ZZhE/FlHuLHk JlA6cbf5GrzAQq1aJE2kXvjz/M3mxFgzeH6hcZy1bg887ih+kWUIH3WEvniS1FUHko f8YGIOhYiEMOuzBgg/da9wIR0wfdVGX9uRtn6A43EsvkQfSSfF9AsfZi03KJ6qA9AP V57RYR+ColbGaPdAWY8sGcIL2XVw1JMdpD2Ue1AuGazkqrgvaG7I6vI4WWh6cmRMvx WUd8LeDNIpmMpgdgOFTk9eXWlfvlOTF6HHD8ezwbFSHAScUReKcXCNxQ1cAR83ZCN1 iiub28M0gMLHQ== Date: Wed, 19 Aug 2026 17:09:13 +0100 From: "Lorenzo Stoakes (ARM)" To: Usama Arif Cc: Andrew Morton , baohua@kernel.org, baolin.wang@linux.alibaba.com, david@kernel.org, dev.jain@arm.com, lance.yang@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, nico.pache@linux.dev, ryan.roberts@arm.com, ziy@nvidia.com, kas@kernel.org, nphamcs@gmail.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kernel-team@meta.com, stable@vger.kernel.org Subject: Re: [PATCH] mm/huge_memory: transfer the pmd dirty bit to the folio on zap Message-ID: References: <20260819101222.3732660-1-usama.arif@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260819101222.3732660-1-usama.arif@linux.dev> On Wed, Aug 19, 2026 at 03:12:22AM -0700, Usama Arif wrote: > zap_huge_pmd_folio() propagates the pmd young bit to the folio for the > file case, but not the dirty bit. The pte path does propagate it, in > zap_present_folio_ptes() and so does the pmd split path, in > __split_huge_pmd_locked(). > > For most file mappings the omission is harmless, because writing to a > shared file mapping goes through page_mkwrite(), which dirties the > folio. tmpfs is different: it has no page_mkwrite(), and > vma_wants_writenotify() is false for it, so a *read* fault on a > MAP_SHARED tmpfs mapping installs a writable pmd via do_read_fault(). > do_read_fault() does not call fault_dirty_shared_page(), so subsequent > stores through that mapping set only the hardware dirty bit in the pmd > and never call folio_mark_dirty(). A shmem folio allocated by a fault > is marked uptodate but not dirty (see the clear: block in > shmem_get_folio_gfp()), so PG_dirty is never set at all. > > Unmapping such a folio - munmap(), or exit_mmap() when the process dies > - then loses the only record that it was written, because zap_huge_pmd() > drops the pmd without transferring the dirty bit. Reclaim afterwards > sees a clean shmem folio: the whole swap-out block in > shrink_folio_list() is inside "if (folio_test_dirty(folio))", so > pageout() is skipped and the folio falls into __remove_mapping(). > There, folio_is_file_lru() is false for a swapbacked folio, so no shadow > entry is created and __filemap_remove_folio(folio, NULL) simply empties > the i_pages slot. The data is freed without ever being written to swap, > and the next fault on that index returns a freshly zeroed folio. > > This is silent data loss for any process that keeps state in a > MAP_SHARED tmpfs segment across an unmap - for example a cache handed > from one process generation to the next through /dev/shm. It requires > the folio to be PMD-mapped, so it only shows up once shmem THP is > enabled (which is what we did in Meta fleet and started noticing crashes); > with THP off the pte path transfers the dirty bit correctly. > It also only becomes visible when swap is enabled, because with no swap > device shmem folios (which are on the anon LRU) are not scanned by > reclaim at all, so the clean folio is never dropped. Good lord. > > Reproduced on x86_64 with a tmpfs mounted huge=within_size: read-fault a > 2MB-backed region, write a known pattern through the resulting mapping, > munmap, force reclaim of the cgroup, then re-map and read back. Without > this patch the region reads back as zeros and vmstat shows zswpout 0 - > the data was discarded rather than swapped. With this patch the region > reads back correctly and the pages are swapped out as expected. With > huge=never, or when the first touch is a write, the test passes either > way. > > Fixes: 800d8c63b2e9 ("shmem: add huge pages support") > Cc: > Signed-off-by: Usama Arif Thanks for this! LGTM so: Reviewed-by: Lorenzo Stoakes (ARM) > --- > mm/huge_memory.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index ced400f72d43a..afbb5974bd225 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -2449,6 +2449,8 @@ static void zap_huge_pmd_folio(struct mm_struct *mm, struct vm_area_struct *vma, > add_mm_counter(mm, mm_counter_file(folio), > -HPAGE_PMD_NR); > > + if (is_present && pmd_dirty(pmdval)) > + folio_mark_dirty(folio); > if (is_present && pmd_young(pmdval) && > likely(vma_has_recency(vma))) > folio_mark_accessed(folio); > -- > 2.53.0-Meta > -- Cheers, Lorenzo