I generated the attached kernel module + userspace utility reproducer to
verify that the issue is indeed resolved in noble/linux 6.8.0-147.147
and resolute/linux 7.0.0-39.39.
With the reproducer it can be seen that the fast path is taken in the
NULL-mapping order-0 folio case now that this patch is applied. The fast
path is usable 0/512 attempts on the unpatched kernels, but can be used
on 512/512 attempts on the patched kernels. Running the reproducer on
noble/linux and resolute/linux yields the following:
noble/linux 6.8.0-146-generic
$ ./gupbench -p 512 -r 5 -i 200 1 8 32 128
page size: 4K, pages/thread: 512, reps: 5, iters: 200
get_user_pages_fast_only() verdict:
gupfast (alloc_page+vm_insert_page): 0/512
anon (control) : 512/512
pin_user_pages_fast(FOLL_WRITE) median ns/page:
threads gupfast anon
1 187.1 110.2
8 171.2 73.1
32 310.4 108.7
128 1212.5 195.2
RESULT: FAIL - GUP-fast rejects NULL-mapping order-0 folios (bug
present)
noble/linux 6.8.0-147-generic
$ ./gupbench -p 512 -r 5 -i 200 1 8 32 128
page size: 4K, pages/thread: 512, reps: 5, iters: 200
get_user_pages_fast_only() verdict:
gupfast (alloc_page+vm_insert_page): 512/512
anon (control) : 512/512
pin_user_pages_fast(FOLL_WRITE) median ns/page:
threads gupfast anon
1 110.2 113.3
8 89.9 74.1
32 114.7 106.6
128 177.9 209.3
RESULT: PASS - GUP-fast accepts NULL-mapping order-0 folios (fixed)
resolute/linux 7.0.0-38-generic
$ ./gupbench -p 512 -r 5 -i 200 1 8 32 128
page size: 4K, pages/thread: 512, reps: 5, iters: 200
get_user_pages_fast_only() verdict:
gupfast (alloc_page+vm_insert_page): 0/512
anon (control) : 512/512
pin_user_pages_fast(FOLL_WRITE) median ns/page:
threads gupfast anon
1 177.3 115.7
8 153.7 117.0
32 257.4 119.5
128 752.7 213.2
RESULT: FAIL - GUP-fast rejects NULL-mapping order-0 folios (bug
present)
resolute/linux 7.0.0-39-generic
$ ./gupbench -p 512 -r 5 -i 200 1 8 32 128
page size: 4K, pages/thread: 512, reps: 5, iters: 200
get_user_pages_fast_only() verdict:
gupfast (alloc_page+vm_insert_page): 512/512
anon (control) : 512/512
pin_user_pages_fast(FOLL_WRITE) median ns/page:
threads gupfast anon
1 121.9 117.1
8 93.8 84.2
32 124.2 115.6
128 155.0 212.5
RESULT: PASS - GUP-fast accepts NULL-mapping order-0 folios (fixed)
** Attachment added: "lp2162917-gup-fast-reproducer.tar.xz"
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2162917/+attachment/6007403/+files/lp2162917-gup-fast-reproducer.tar.xz
** Tags removed: verification-needed-noble-linux
verification-needed-resolute-linux verification-needed-resolute-linux-nvidia
verification-needed-resolute-linux-nvidia-bos
** Tags added: verification-done-noble-linux verification-done-resolute-linux
verification-done-resolute-linux-nvidia
verification-done-resolute-linux-nvidia-bos
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https://bugs.launchpad.net/bugs/2162917
Title:
Backport: "mm/gup: fix GUP-fast fallback for NULL-mapping order-0
folios"
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