On 19.02.25 21:37, Barry Song wrote:
On Thu, Feb 20, 2025 at 7:27 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
From: Barry Song <v-songbaohua@xxxxxxxx>
userfaultfd_move() checks whether the PTE entry is present or a
swap entry.
- If the PTE entry is present, move_present_pte() handles folio
migration by setting:
src_folio->index = linear_page_index(dst_vma, dst_addr);
- If the PTE entry is a swap entry, move_swap_pte() simply copies
the PTE to the new dst_addr.
This approach is incorrect because even if the PTE is a swap
entry, it can still reference a folio that remains in the swap
cache.
If do_swap_page() is triggered, it may locate the folio in the
swap cache. However, during add_rmap operations, a kernel panic
can occur due to:
page_pgoff(folio, page) != linear_page_index(vma, address)
Thanks for the report and reproducer!
$./a.out > /dev/null
[ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c
[ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0
[ 13.337716] memcg:ffff00000405f000
[ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff)
[ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
[ 13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
[ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
[ 13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
[ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001
[ 13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000
[ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address))
[ 13.340190] ------------[ cut here ]------------
[ 13.340316] kernel BUG at mm/rmap.c:1380!
[ 13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
[ 13.340969] Modules linked in:
[ 13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299
[ 13.341470] Hardware name: linux,dummy-virt (DT)
[ 13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 13.341815] pc : __page_check_anon_rmap+0xa0/0xb0
[ 13.341920] lr : __page_check_anon_rmap+0xa0/0xb0
[ 13.342018] sp : ffff80008752bb20
[ 13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001
[ 13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001
[ 13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00
[ 13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff
[ 13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f
[ 13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0
[ 13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40
[ 13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8
[ 13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000
[ 13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f
[ 13.343876] Call trace:
[ 13.344045] __page_check_anon_rmap+0xa0/0xb0 (P)
[ 13.344234] folio_add_anon_rmap_ptes+0x22c/0x320
[ 13.344333] do_swap_page+0x1060/0x1400
[ 13.344417] __handle_mm_fault+0x61c/0xbc8
[ 13.344504] handle_mm_fault+0xd8/0x2e8
[ 13.344586] do_page_fault+0x20c/0x770
[ 13.344673] do_translation_fault+0xb4/0xf0
[ 13.344759] do_mem_abort+0x48/0xa0
[ 13.344842] el0_da+0x58/0x130
[ 13.344914] el0t_64_sync_handler+0xc4/0x138
[ 13.345002] el0t_64_sync+0x1ac/0x1b0
[ 13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000)
[ 13.345504] ---[ end trace 0000000000000000 ]---
[ 13.345715] note: a.out[107] exited with irqs disabled
[ 13.345954] note: a.out[107] exited with preempt_count 2
Fully fixing it would be quite complex, requiring similar handling
of folios as done in move_present_pte.
How complex would that be? Is it a matter of adding
folio_maybe_dma_pinned() checks, doing folio_move_anon_rmap() and
folio->index = linear_page_index like in move_present_pte() or
something more?
My main concern is still with large folios that require a split_folio()
during move_pages(), as the entire folio shares the same index and
anon_vma. However, userfaultfd_move() moves pages individually,
making a split necessary.
However, in split_huge_page_to_list_to_order(), there is a:
if (folio_test_writeback(folio))
return -EBUSY;
This is likely true for swapcache, right? However, even for move_present_pte(),
it simply returns -EBUSY:
move_pages_pte()
{
/* at this point we have src_folio locked */
if (folio_test_large(src_folio)) {
/* split_folio() can block */
pte_unmap(&orig_src_pte);
pte_unmap(&orig_dst_pte);
src_pte = dst_pte = NULL;
err = split_folio(src_folio);
if (err)
goto out;
/* have to reacquire the folio after it got split */
folio_unlock(src_folio);
folio_put(src_folio);
src_folio = NULL;
goto retry;
}
}
Do we need a folio_wait_writeback() before calling split_folio()?
By the way, I have also reported that userfaultfd_move() has a fundamental
conflict with TAO (Cc'ed Yu Zhao), which has been part of the Android common
kernel. In this scenario, folios in the virtual zone won’t be split in
split_folio(). Instead, the large folio migrates into nr_pages small folios.
> > Thus, the best-case scenario would be:
mTHP -> migrate to small folios in split_folio() -> move small folios to
dst_addr
While this works, it negates the performance benefits of
userfaultfd_move(), as it introduces two PTE operations (migration in
split_folio() and move in userfaultfd_move() while retry), nr_pages memory
allocations, and still requires one memcpy(). This could end up
performing even worse than userfaultfd_copy(), I guess.
> > The worst-case scenario would be failing to allocate small folios in
split_folio(), then userfaultfd_move() might return -ENOMEM?
Although that's an Android problem and not an upstream problem, I'll
note that there are other reasons why the split / move might fail, and
user space either must retry or fallback to a COPY.
Regarding mTHP, we could move the whole folio if the user space-provided
range allows for batching over multiple PTEs (nr_ptes), they are in a
single VMA, and folio_mapcount() == nr_ptes.
There are corner cases to handle, such as moving mTHPs such that they
suddenly cross two page tables I assume, that are harder to handle when
not moving individual PTEs where that cannot happen.
--
Cheers,
David / dhildenb
