Re: [PATCH v4 13/13] mm/gup: Handle hugetlb in the generic follow_page_mask code
From: Peter Xu
Date: Tue Apr 02 2024 - 12:20:28 EST
On Tue, Apr 02, 2024 at 05:26:28PM +0200, David Hildenbrand wrote:
> On 02.04.24 16:48, Ryan Roberts wrote:
> > Hi Peter,
Hey, Ryan,
Thanks for the report!
> >
> > On 27/03/2024 15:23, peterx@xxxxxxxxxx wrote:
> > > From: Peter Xu <peterx@xxxxxxxxxx>
> > >
> > > Now follow_page() is ready to handle hugetlb pages in whatever form, and
> > > over all architectures. Switch to the generic code path.
> > >
> > > Time to retire hugetlb_follow_page_mask(), following the previous
> > > retirement of follow_hugetlb_page() in 4849807114b8.
> > >
> > > There may be a slight difference of how the loops run when processing slow
> > > GUP over a large hugetlb range on cont_pte/cont_pmd supported archs: each
> > > loop of __get_user_pages() will resolve one pgtable entry with the patch
> > > applied, rather than relying on the size of hugetlb hstate, the latter may
> > > cover multiple entries in one loop.
> > >
> > > A quick performance test on an aarch64 VM on M1 chip shows 15% degrade over
> > > a tight loop of slow gup after the path switched. That shouldn't be a
> > > problem because slow-gup should not be a hot path for GUP in general: when
> > > page is commonly present, fast-gup will already succeed, while when the
> > > page is indeed missing and require a follow up page fault, the slow gup
> > > degrade will probably buried in the fault paths anyway. It also explains
> > > why slow gup for THP used to be very slow before 57edfcfd3419 ("mm/gup:
> > > accelerate thp gup even for "pages != NULL"") lands, the latter not part of
> > > a performance analysis but a side benefit. If the performance will be a
> > > concern, we can consider handle CONT_PTE in follow_page().
> > >
> > > Before that is justified to be necessary, keep everything clean and simple.
> > >
> > > Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> >
> > Afraid I'm seeing an oops when running gup_longterm test on arm64 with current mm-unstable. Git bisect blames this patch. The oops reproduces for me every time on 2 different machines:
> >
> >
> > [ 9.340416] kernel BUG at mm/gup.c:778!
> > [ 9.340746] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
> > [ 9.341199] Modules linked in:
> > [ 9.341481] CPU: 1 PID: 1159 Comm: gup_longterm Not tainted 6.9.0-rc2-00210-g910ff1a347e4 #11
> > [ 9.342232] Hardware name: linux,dummy-virt (DT)
> > [ 9.342647] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > [ 9.343195] pc : follow_page_mask+0x4d4/0x880
> > [ 9.343580] lr : follow_page_mask+0x4d4/0x880
> > [ 9.344018] sp : ffff8000898b3aa0
> > [ 9.344345] x29: ffff8000898b3aa0 x28: fffffdffc53973e8 x27: 00003c0005d08000
> > [ 9.345028] x26: ffff00014e5cfd08 x25: ffffd3513a40c000 x24: fffffdffc5d08000
> > [ 9.345682] x23: ffffc1ffc0000000 x22: 0000000000080101 x21: ffff8000898b3ba8
> > [ 9.346337] x20: 0000fffff4200000 x19: ffff00014e52d508 x18: 0000000000000010
> > [ 9.347005] x17: 5f656e6f7a5f7369 x16: 2120262620296567 x15: 6170286461654865
> > [ 9.347713] x14: 6761502128454741 x13: 2929656761702865 x12: 6761705f65636976
> > [ 9.348371] x11: 65645f656e6f7a5f x10: ffffd3513b31d6e0 x9 : ffffd3513852f090
> > [ 9.349062] x8 : 00000000ffffefff x7 : ffffd3513b31d6e0 x6 : 0000000000000000
> > [ 9.349753] x5 : ffff00017ff98cc8 x4 : 0000000000000fff x3 : 0000000000000000
> > [ 9.350397] x2 : 0000000000000000 x1 : ffff000190e8b480 x0 : 0000000000000052
> > [ 9.351097] Call trace:
> > [ 9.351312] follow_page_mask+0x4d4/0x880
> > [ 9.351700] __get_user_pages+0xf4/0x3e8
> > [ 9.352089] __gup_longterm_locked+0x204/0xa70
> > [ 9.352516] pin_user_pages+0x88/0xc0
> > [ 9.352873] gup_test_ioctl+0x860/0xc40
> > [ 9.353249] __arm64_sys_ioctl+0xb0/0x100
> > [ 9.353648] invoke_syscall+0x50/0x128
> > [ 9.354022] el0_svc_common.constprop.0+0x48/0xf8
> > [ 9.354488] do_el0_svc+0x28/0x40
> > [ 9.354822] el0_svc+0x34/0xe0
> > [ 9.355128] el0t_64_sync_handler+0x13c/0x158
> > [ 9.355489] el0t_64_sync+0x190/0x198
> > [ 9.355793] Code: aa1803e0 d000d8e1 91220021 97fff560 (d4210000)
> > [ 9.356280] ---[ end trace 0000000000000000 ]---
> > [ 9.356651] note: gup_longterm[1159] exited with irqs disabled
> > [ 9.357141] note: gup_longterm[1159] exited with preempt_count 2
> > [ 9.358033] ------------[ cut here ]------------
> > [ 9.358800] WARNING: CPU: 1 PID: 0 at kernel/context_tracking.c:128 ct_kernel_exit.constprop.0+0x108/0x120
> > [ 9.360157] Modules linked in:
> > [ 9.360541] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G D 6.9.0-rc2-00210-g910ff1a347e4 #11
> > [ 9.361626] Hardware name: linux,dummy-virt (DT)
> > [ 9.362087] pstate: 204003c5 (nzCv DAIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > [ 9.362758] pc : ct_kernel_exit.constprop.0+0x108/0x120
> > [ 9.363306] lr : ct_idle_enter+0x10/0x20
> > [ 9.363845] sp : ffff8000801abdc0
> > [ 9.364222] x29: ffff8000801abdc0 x28: 0000000000000000 x27: 0000000000000000
> > [ 9.364961] x26: 0000000000000000 x25: ffff00014149d780 x24: 0000000000000000
> > [ 9.365557] x23: 0000000000000000 x22: ffffd3513b299d48 x21: ffffd3513a785730
> > [ 9.366239] x20: ffffd3513b299c28 x19: ffff00017ffa7da0 x18: 0000fffff5ffffff
> > [ 9.366869] x17: 0000000000000000 x16: 1fffe0002a21a8c1 x15: 0000000000000000
> > [ 9.367524] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000002
> > [ 9.368207] x11: 0000000000000001 x10: 0000000000000ad0 x9 : ffffd35138589230
> > [ 9.369123] x8 : ffff00014149e2b0 x7 : 0000000000000000 x6 : 000000000f8c0fb2
> > [ 9.370403] x5 : 4000000000000002 x4 : ffff2cb045825000 x3 : ffff8000801abdc0
> > [ 9.371170] x2 : ffffd3513a782da0 x1 : 4000000000000000 x0 : ffffd3513a782da0
> > [ 9.372279] Call trace:
> > [ 9.372519] ct_kernel_exit.constprop.0+0x108/0x120
> > [ 9.373216] ct_idle_enter+0x10/0x20
> > [ 9.373562] default_idle_call+0x3c/0x160
> > [ 9.374055] do_idle+0x21c/0x280
> > [ 9.374394] cpu_startup_entry+0x3c/0x50
> > [ 9.374797] secondary_start_kernel+0x140/0x168
> > [ 9.375220] __secondary_switched+0xb8/0xc0
> > [ 9.375875] ---[ end trace 0000000000000000 ]---
> >
> >
> > The oops trigger is at mm/gup.c:778:
> > VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
> >
> >
> > This is the output of gup_longterm (last output is just before oops):
> >
> > # [INFO] detected hugetlb page size: 2048 KiB
> > # [INFO] detected hugetlb page size: 32768 KiB
> > # [INFO] detected hugetlb page size: 64 KiB
> > # [INFO] detected hugetlb page size: 1048576 KiB
> > TAP version 13
> > 1..70
> > # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd
> > ok 1 Should have worked
> > # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with tmpfile
> > ok 2 Should have failed
> > # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with local tmpfile
> > ok 3 Should have failed
> > # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB)
> > ok 4 Should have worked
> > # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (32768 kB)
> >
> >
> > So 2M passed ok, and its failing for 32M, which is cont-pmd. I'm guessing you're trying to iterate 2M into a cont-pmd folio and ending up with an unexpected tail page?
>
> I assume we find the expected tail page, it's just that the check
>
> VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
>
> Doesn't make sense with hugetlb folios. We might have a tail page mapped in
> a cont-pmd entry. As soon as we call follow_huge_pmd() on "not the first
> cont-pmd entry", we trigger this check.
>
> Likely this sanity check must also allow for hugetlb folios. Or we should
> just remove it completely.
Right, IMHO it'll be easier we remove it, actually I see there's one more
at the end, so I think we need to remove both.
>
> In the past, we wanted to make sure that we never get tail pages of THP from
> PMD entries, because something would currently be broken (we don't support
> THP > PMD).
There's probably one more thing we need to do, on allowing
PageAnonExclusive() to work with hugetlb tails. Even if we remove the
warnings and if I read the code right, we can BUG_ON again on checking tail
pages over anon-exclusive for PageHuge.
So I assume to fix it completely, we may need two changes: Patch 1 to
prepare PageAnonExclusive() to work on hugetlb tails, then patch 2 to be
squashed into the patch "mm/gup: handle huge pmd for follow_pmd_mask()".
Note: not this patch to fixup, as this patch only does the "switchover" to
the new path, the culprit should be the other patch..
I have them attached below first, before I'll also go and see whether I can
run some arm tests later today or tomorrow. David, any comments from
anon-exclusive side?
Thanks,
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