Re: [PATCH v3] mm/hugetlb: fix hugetlb vs. core-mm PT locking

[Christophe Leroy <christophe.leroy@xxxxxxxxxx>]

Le 31/07/2024 à 18:33, David Hildenbrand a écrit :
> On 31.07.24 16:54, Peter Xu wrote:
> > On Wed, Jul 31, 2024 at 02:21:03PM +0200, David Hildenbrand wrote:
> > > We recently made GUP's common page table walking code to also walk 
> > > hugetlb
> > > VMAs without most hugetlb special-casing, preparing for the future of
> > > having less hugetlb-specific page table walking code in the codebase.
> > > Turns out that we missed one page table locking detail: page table 
> > > locking
> > > for hugetlb folios that are not mapped using a single PMD/PUD.
> > >
> > > Assume we have hugetlb folio that spans multiple PTEs (e.g., 64 KiB
> > > hugetlb folios on arm64 with 4 KiB base page size). GUP, as it walks the
> > > page tables, will perform a pte_offset_map_lock() to grab the PTE table
> > > lock.
> > >
> > > However, hugetlb that concurrently modifies these page tables would
> > > actually grab the mm->page_table_lock: with USE_SPLIT_PTE_PTLOCKS, the
> > > locks would differ. Something similar can happen right now with hugetlb
> > > folios that span multiple PMDs when USE_SPLIT_PMD_PTLOCKS.
> > >
> > > This issue can be reproduced [1], for example triggering:
> > >
> > > [ 3105.936100] ------------[ cut here ]------------
> > > [ 3105.939323] WARNING: CPU: 31 PID: 2732 at mm/gup.c:142 
> > > try_grab_folio+0x11c/0x188
> > > [ 3105.944634] Modules linked in: [...]
> > > [ 3105.974841] CPU: 31 PID: 2732 Comm: reproducer Not tainted 
> > > 6.10.0-64.eln141.aarch64 #1
> > > [ 3105.980406] Hardware name: QEMU KVM Virtual Machine, BIOS 
> > > edk2-20240524-4.fc40 05/24/2024
> > > [ 3105.986185] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS 
> > > BTYPE=--)
> > > [ 3105.991108] pc : try_grab_folio+0x11c/0x188
> > > [ 3105.994013] lr : follow_page_pte+0xd8/0x430
> > > [ 3105.996986] sp : ffff80008eafb8f0
> > > [ 3105.999346] x29: ffff80008eafb900 x28: ffffffe8d481f380 x27: 
> > > 00f80001207cff43
> > > [ 3106.004414] x26: 0000000000000001 x25: 0000000000000000 x24: 
> > > ffff80008eafba48
> > > [ 3106.009520] x23: 0000ffff9372f000 x22: ffff7a54459e2000 x21: 
> > > ffff7a546c1aa978
> > > [ 3106.014529] x20: ffffffe8d481f3c0 x19: 0000000000610041 x18: 
> > > 0000000000000001
> > > [ 3106.019506] x17: 0000000000000001 x16: ffffffffffffffff x15: 
> > > 0000000000000000
> > > [ 3106.024494] x14: ffffb85477fdfe08 x13: 0000ffff9372ffff x12: 
> > > 0000000000000000
> > > [ 3106.029469] x11: 1fffef4a88a96be1 x10: ffff7a54454b5f0c x9 : 
> > > ffffb854771b12f0
> > > [ 3106.034324] x8 : 0008000000000000 x7 : ffff7a546c1aa980 x6 : 
> > > 0008000000000080
> > > [ 3106.038902] x5 : 00000000001207cf x4 : 0000ffff9372f000 x3 : 
> > > ffffffe8d481f000
> > > [ 3106.043420] x2 : 0000000000610041 x1 : 0000000000000001 x0 : 
> > > 0000000000000000
> > > [ 3106.047957] Call trace:
> > > [ 3106.049522]  try_grab_folio+0x11c/0x188
> > > [ 3106.051996]  follow_pmd_mask.constprop.0.isra.0+0x150/0x2e0
> > > [ 3106.055527]  follow_page_mask+0x1a0/0x2b8
> > > [ 3106.058118]  __get_user_pages+0xf0/0x348
> > > [ 3106.060647]  faultin_page_range+0xb0/0x360
> > > [ 3106.063651]  do_madvise+0x340/0x598
> > >
> > > Let's make huge_pte_lockptr() effectively use the same PT locks as any
> > > core-mm page table walker would. Add ptep_lockptr() to obtain the PTE
> > > page table lock using a pte pointer -- unfortunately we cannot convert
> > > pte_lockptr() because virt_to_page() doesn't work with kmap'ed page
> > > tables we can have with CONFIG_HIGHPTE.
> > >
> > > Take care of PTE tables possibly spanning multiple pages, and take 
> > > care of
> > > CONFIG_PGTABLE_LEVELS complexity when e.g., PMD_SIZE == PUD_SIZE. For
> > > example, with CONFIG_PGTABLE_LEVELS == 2, core-mm would detect
> > > with hugepagesize==PMD_SIZE pmd_leaf() and use the pmd_lockptr(), which
> > > would end up just mapping to the per-MM PT lock.
> > >
> > > There is one ugly case: powerpc 8xx, whereby we have an 8 MiB hugetlb
> > > folio being mapped using two PTE page tables.  While hugetlb wants to 
> > > take
> > > the PMD table lock, core-mm would grab the PTE table lock of one of both
> > > PTE page tables.  In such corner cases, we have to make sure that both
> > > locks match, which is (fortunately!) currently guaranteed for 8xx as it
> > > does not support SMP and consequently doesn't use split PT locks.
> > >
> > > [1] 
> > > https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fall%2F1bbfcc7f-f222-45a5-ac44-c5a1381c596d%40redhat.com%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cf91a0b3cdcab46c7bd6108dcb17e9454%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638580404425532305%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=%2FQ4QFqbyThojISHACwzxCdtYbgwc4JsMIP%2Bmx4PneOk%3D&reserved=0
> > >
> > > Fixes: 9cb28da54643 ("mm/gup: handle hugetlb in the generic 
> > > follow_page_mask code")
> > > Reviewed-by: James Houghton <jthoughton@xxxxxxxxxx>
> > > Cc: <stable@xxxxxxxxxxxxxxx>
> > > Cc: Peter Xu <peterx@xxxxxxxxxx>
> > > Cc: Oscar Salvador <osalvador@xxxxxxx>
> > > Cc: Muchun Song <muchun.song@xxxxxxxxx>
> > > Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >
> > Nitpick: I wonder whether some of the lines can be simplified if we write
> > it downwards from PUD, like,
> >
> > huge_pte_lockptr()
> > {
> >          if (size >= PUD_SIZE)
> >             return pud_lockptr(...);
> >          if (size >= PMD_SIZE)
> >             return pmd_lockptr(...);
> >          /* Sub-PMD only applies to !CONFIG_HIGHPTE, see 
> > pte_alloc_huge() */
> >          WARN_ON(IS_ENABLED(CONFIG_HIGHPTE));
> >          return ptep_lockptr(...);
> > }
>
> Let me think about it. For PUD_SIZE == PMD_SIZE instead of like core-mm
> calling pmd_lockptr we'd call pud_lockptr().

I guess it is only when including asm-generic/pgtable-nopmd.h

Otherwise you should have more than one entry in the PMD table so 
PMD_SIZE would always be smaller than PUD_SIZE, wouldn't it ?

So maybe some simplification could be done, like having pud_lock() a nop 
in that case ?


Christophe