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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 20 Mar 2025 13:33:36.8319 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 5c84543d-8fc7-4f09-b254-08dd67b3d18c X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[63.35.35.123];Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: AMS1EPF0000004A.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DU0PR08MB9822 On Mon, Mar 17, 2025 at 09:15:02PM +0000, Lorenzo Stoakes wrote: > It appears that we have been incorrectly rejecting merge cases for 15 > years, apparently by mistake. > > Imagine a range of anonymous mapped momemory divided into two VMAs like > this, with incompatible protection bits: > > RW RWX > unfaulted faulted > |-----------|-----------| > | prev | vma | > |-----------|-----------| > mprotect(RW) > > Now imagine mprotect()'ing vma so it is RW. This appears as if it should > merge, it does not. > > Neither does this case, again mprotect()'ing vma RW: > > RWX RW > faulted unfaulted > |-----------|-----------| > | vma | next | > |-----------|-----------| > mprotect(RW) > > Nor: > > RW RWX RW > unfaulted faulted unfaulted > |-----------|-----------|-----------| > | prev | vma | next | > |-----------|-----------|-----------| > mprotect(RW) > > What's going on here? > > In commit 5beb49305251 ("mm: change anon_vma linking to fix multi-process > server scalability issue"), from 2010, Rik von Riel took careful care to > account for these cases - commenting that '[this is] easily overlooked: > when mprotect shifts the boundary, make sure the expanding vma has anon_vma > set if the shrinking vma had, to cover any anon pages imported.' > > However, commit 965f55dea0e3 ("mmap: avoid merging cloned VMAs") introduced > a little over a year later, appears to have accidentally disallowed this. > > By adjusting the is_mergeable_anon_vma() function to avoid lock contention > across large trees of forked anon_vma's, this commit wrongly assumed the > VMA being checked (the ostensible merge 'target') should be faulted, that > is, have an anon_vma, and thus an anon_vma_chain list established, but only > of length 1. > > This appears to have been unintentional, as disallowing empty target VMAs > like this across the board makes no sense. > > We already have logic that accounts for this case, the same logic Rik > introduced in 2010, now via dup_anon_vma() (and ultimately > anon_vma_clone()), so there is no problem permitting this. > > This series fixes this mistake and also ensures that scalability concerns > remain addressed by explicitly checking that whatever VMA is being merged > has not been forked. > > A full set of self tests which reproduce the issue are provided, as well as > updating userland VMA tests to assert this behaviour. > > The self tests additionally assert scalability concerns are addressed. > > > Lorenzo Stoakes (3): > mm/vma: fix incorrectly disallowed anonymous VMA merges > tools/testing: add PROCMAP_QUERY helper functions in mm self tests > tools/testing/selftests: assert that anon merge cases behave as > expected > > mm/vma.c | 81 ++-- > tools/testing/selftests/mm/.gitignore | 1 + > tools/testing/selftests/mm/Makefile | 1 + > tools/testing/selftests/mm/merge.c | 454 ++++++++++++++++++++++ > tools/testing/selftests/mm/run_vmtests.sh | 2 + > tools/testing/selftests/mm/vm_util.c | 62 +++ > tools/testing/selftests/mm/vm_util.h | 21 + > tools/testing/vma/vma.c | 100 ++--- > 8 files changed, 652 insertions(+), 70 deletions(-) > create mode 100644 tools/testing/selftests/mm/merge.c > > -- > 2.48.1 > Look good to me. Reviewed-by: Yeoreum Yun