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 30CC448E0ED for ; Tue, 8 Sep 2026 19:50:35 +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=1788897038; cv=none; b=bTaN2/O+TYQ15KbMfAHjE855gYS2+sV/9SNqfemIcwCCqBiuLT71vb6/xXD06QyGJqo9MpCQXBKhHdYB1PPrLx2Iopwm8Q08cPRDmAqglrkbjOdcXQAE5nbdBp2BJusL4Vo3wnuZBs6TKhCy+HVP9UtGTfk7NOscZAjCgYI9S7k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788897038; c=relaxed/simple; bh=xqjVqpDaNTc0sfdg/n6ePRss4yF40sfCOX5xDAx+IBg=; h=Date:To:From:Subject:Message-Id; b=Z8AvX77vIjc0KGwXWYH68PkmSC+XxNB0aGFqG/Z9zfc70atucf21M8x2hNoqR9LwZuoJOEGYcRqK/mdLOYF9+HuIAtaZErSaeI8ypYQd6bQo2T8P4eevOSfDjFmZ5HGhy0vJCA/q6QfDubxDlIg3wZ8PcC5nvSmhtLtJxg8huh0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=UyTrqH72; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="UyTrqH72" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D34F31F00A3A; Tue, 8 Sep 2026 19:50:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1788897034; bh=LHskbwg9FxtexeTW3lTVUxJ4PQ9ixAfmrvV3zE+XM6A=; h=Date:To:From:Subject; b=UyTrqH72A2NLcBWXOHUZ8RC+Wucz8L3XuueOtKZpn6O1/o62wqJ7HlPAF4AD54c1L aPNAE4SY57TWOaXQxcL+WNdywWBd3ErJyzZiaYctw+CbpS67MrfCeQ9biFYgwF/VNG uZapTzuyAdeVmy/eo1VUdVHBuzLSD8LEZ7wgnbEs= Date: Tue, 08 Sep 2026 12:50:34 -0700 To: mm-commits@vger.kernel.org,kas@kernel.org,akpm@linux-foundation.org From: Andrew Morton Subject: + selftests-mm-cover-a-shared-source-collapse-write-race.patch added to mm-new branch Message-Id: <20260908195034.D34F31F00A3A@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: selftests/mm: cover a shared-source collapse write race has been added to the -mm mm-new branch. Its filename is selftests-mm-cover-a-shared-source-collapse-write-race.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-mm-cover-a-shared-source-collapse-write-race.patch This patch will later appear in the mm-new branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Note, mm-new is a provisional staging ground for work-in-progress patches, and acceptance into mm-new is a notification for others take notice and to finish up reviews. Please do not hesitate to respond to review feedback and post updated versions to replace or incrementally fixup patches in mm-new. The mm-new branch of mm.git is not included in linux-next If a few days of testing in mm-new is successful, the patch will me moved into mm.git's mm-unstable branch, which is included in linux-next Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: "Kiryl Shutsemau (Meta)" Subject: selftests/mm: cover a shared-source collapse write race Date: Tue, 8 Sep 2026 13:50:59 +0100 collapse_fork() checks that a fork-shared range collapses in the child while the parent keeps its own pages, but the parent sits still while that happens. Nothing checks that CoW isolation survives a collapse racing with writes to the shared source. Add a case where the parent writes to the shared range throughout the child's collapse. CoW has to keep the two apart: the child must see the content from before the fork, and the parent only its own writes. The parent unshares one page every 10ms, starting only once the child says it is about to collapse. Writing the range in a burst would break CoW on all of it before the collapse begins, leaving the child to collapse pages that are already exclusive to it. Preparation for changing how collapse handles fork-shared sources. Assisted-by: LLM Link: https://lore.kernel.org/20260908125105.1510704-14-kirill@shutemov.name Signed-off-by: Kiryl Shutsemau (Meta) Tested-by: Muhammad Usama Anjum Cc: Alexander Gordeev Cc: Baolin Wang Cc: Barry Song Cc: David Hildenbrand Cc: Dev Jain Cc: Hugh Dickins Cc: Jason Gunthorpe Cc: Lance Yang Cc: Leon Romanovsky Cc: Liam R. Howlett Cc: Lorenzo Stoakes (ARM) Cc: Michal Hocko Cc: Mike Rapoport (Microsoft) Cc: Ryan Roberts Cc: Shuah Khan Cc: Suren Baghdasaryan Cc: Usama Arif Cc: Vlastimil Babka Cc: Zi Yan Signed-off-by: Andrew Morton --- tools/testing/selftests/mm/khugepaged.c | 100 ++++++++++++++++++++++ 1 file changed, 100 insertions(+) --- a/tools/testing/selftests/mm/khugepaged.c~selftests-mm-cover-a-shared-source-collapse-write-race +++ a/tools/testing/selftests/mm/khugepaged.c @@ -1169,6 +1169,103 @@ static void collapse_max_ptes_shared(str ksft_test_result_report(exit_status, "%s\n", __func__); } +/* + * The parent writes to the fork-shared range throughout the child's + * collapse. CoW must keep the two apart: the child sees the pre-fork + * content, the parent only its own writes. + */ +static void collapse_fork_cow_race(struct collapse_context *c, struct mem_ops *ops) +{ + const int stride = page_size / sizeof(int); + int wstatus, child_status, i, n; + unsigned long shared; + volatile int *ip; + pid_t child; + int sync[2]; + char go = 1; + void *p; + + /* At a page per 10 ms, 64 pages spread the writes across the collapse */ + n = 64; + shared = n * page_size; + + p = ops->setup_area(1); + /* Shared prefix, with the pre-fork pattern */ + ops->fault(p, 0, shared); + if (pipe(sync)) + ksft_exit_fail_perror("pipe()"); + + /* A volatile pointer so the stores are not merged or dropped */ + ip = p; + + ksft_print_msg("Fork, collapse in the child while the parent rewrites..."); + child = fork(); + if (!child) { + int collapse_status; + + close(sync[0]); + /* Private remainder */ + ops->fault(p, shared, hpage_pmd_size); + /* Start the parent unsharing, and give it a head start */ + if (write(sync[1], &go, 1) != 1) + _exit(KSFT_FAIL); + usleep(5000); + c->collapse("Collapse a range the parent is writing to", + p, 1, ops, true); + collapse_status = exit_status; + for (i = 0; i < n; i++) + if (ip[i * stride] != i + 0xdead0000) + break; + if (i == n) + success("OK"); + else + fail("Fail: child content"); + /* The content check must not bury a failed collapse */ + if (exit_status != KSFT_FAIL) + exit_status = collapse_status; + ops->cleanup_area(p, hpage_pmd_size); + _exit(exit_status); + } + + close(sync[1]); + if (read(sync[0], &go, 1) != 1) + ksft_exit_fail_msg("child never reached the collapse\n"); + + /* + * Unshare one page at a time: a burst would break CoW on the whole + * range before the collapse starts, leaving nothing shared to collapse. + */ + i = 0; + for (;;) { + if (i < n) + ip[i * stride] = i + 0xbeef0000; + i++; + usleep(10 * 1000); + if (waitpid(child, &wstatus, WNOHANG)) + break; + } + + /* Finish whatever the paced sweep did not reach */ + for (; i < n; i++) + ip[i * stride] = i + 0xbeef0000; + /* A child that died reading the racing pages is a failure, not a zero */ + child_status = WIFEXITED(wstatus) ? WEXITSTATUS(wstatus) : KSFT_FAIL; + + ksft_print_msg("Check the parent sees only its own writes..."); + for (i = 0; i < n; i++) + if (ip[i * stride] != i + 0xbeef0000) + break; + if (i == n) + success("OK"); + else + fail("Fail: parent content"); + ops->cleanup_area(p, hpage_pmd_size); + /* The parent's check must not bury the child's verdict */ + if (exit_status != KSFT_FAIL) + exit_status = child_status; + ksft_test_result_report(exit_status, "%s\n", __func__); +} + static void madvise_collapse_existing_thps(struct collapse_context *c, struct mem_ops *ops) { @@ -1700,6 +1797,9 @@ int main(int argc, char **argv) TEST(collapse_max_ptes_shared, khugepaged_context, anon_ops); TEST(collapse_max_ptes_shared, madvise_context, anon_ops); + TEST(collapse_fork_cow_race, khugepaged_context, anon_ops); + TEST(collapse_fork_cow_race, madvise_context, anon_ops); + TEST(madvise_collapse_existing_thps, madvise_context, anon_ops); TEST(madvise_collapse_existing_thps, madvise_context, read_only_file_ops); TEST(madvise_collapse_existing_thps, madvise_context, read_write_file_read_ops); _ Patches currently in -mm which might be from kas@kernel.org are mm-huge_memory-do-not-touch-frozen-folios-in-deferred_split_isolate.patch mm-huge_memory-dequeue-the-deferred-split-after-the-split-freeze.patch selftests-mm-raise-the-khugepaged-test-case-cap.patch selftests-mm-skip-collapse_compound_extreme-where-the-pmd-is-too-large.patch selftests-mm-scale-khugepageds-collapse-wait-with-the-pmd-size.patch selftests-mm-skip-khugepaged-page-cache-cases-without-a-pmd-folio.patch selftests-mm-make-the-swap-cases-swapout-reliable.patch selftests-mm-stop-khugepaged-during-the-madv_collapse-cases.patch selftests-mm-move-is_backed_by_folio-into-vm_util.patch selftests-mm-add-folio-order-check-for-address-ranges.patch selftests-mm-add-folio-order-detection-self-check.patch selftests-mm-add-khugepaged-completion-barrier-helper.patch selftests-mm-add-order-parameterized-khugepaged-collapse-cases.patch selftests-mm-parameterize-the-mixed-source-collapse-case-by-source-order.patch selftests-mm-cover-a-shared-source-collapse-write-race.patch selftests-mm-run-every-supported-collapse-order-by-default.patch selftests-mm-check-that-one-khugepaged-pass-collapses-one-window.patch selftests-mm-add-khugepaged-race-harness.patch selftests-mm-race-the-collapse-of-windows-with-holes.patch selftests-mm-add-memory-pressure-threads-to-the-khugepaged-race-harness.patch selftests-mm-zap-whole-pte-tables-in-the-khugepaged-race-harness.patch