Message-ID: <1bbfcc7f-f222-45a5-ac44-c5a1381c596d@redhat.com>
Date: Fri, 26 Jul 2024 13:40:23 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>, Peter Xu <peterx@...hat.com>,
 Oscar Salvador <osalvador@...e.de>, stable@...r.kernel.org
Subject: Re: [PATCH v1 2/2] mm/hugetlb: fix hugetlb vs. core-mm PT locking

On 26.07.24 11:38, Baolin Wang wrote:
> 
> 
> On 2024/7/26 16:04, David Hildenbrand wrote:
>> On 26.07.24 04:33, Baolin Wang wrote:
>>>
>>>
>>> On 2024/7/26 02:39, 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.
>>>>
>>>> Let's make huge_pte_lockptr() effectively uses the same PT locks as any
>>>> core-mm page table walker would.
>>>
>>> Thanks for raising the issue again. I remember fixing this issue 2 years
>>> ago in commit fac35ba763ed ("mm/hugetlb: fix races when looking up a
>>> CONT-PTE/PMD size hugetlb page"), but it seems to be broken again.
>>>
>>
>> Ah, right! We fixed it by rerouting to hugetlb code that we then removed :D
>>
>> Did we have a reproducer back then that would make my live easier?
> 
> I don't have any reproducers right now. I remember I added some ugly
> hack code (adding delay() etc.) in kernel to analyze this issue, and not
> easy to reproduce. :(

Hah!

I tried with 32MB without luck -- migration simply takes too long. 64KB
did the trick within seconds!


On a VM with 2 vNUMA nodes, after reserving a bunch of 64KiB hugetlb pages:

# numactl -H
available: 2 nodes (0-1)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
node 0 size: 64439 MB
node 0 free: 64097 MB
node 1 cpus: 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
node 1 size: 64205 MB
node 1 free: 63809 MB
node distances:
node   0   1
   0:  10  20
   1:  20  10
# echo 100  > /sys/kernel/mm/hugepages/hugepages-64kB/nr_hugepagespages-64kB/nr_hugepagesepages-64kB/nr_hugepages
# gcc reproducer.c -o reproducer -O3 -lnuma -lpthread
# ./reproducer
[ 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: nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 rfkill ip_set nf_tables qrtr sunrpc vfat fat ext4 mbcache jbd2 virtio_net net_failover failover virtio_balloon dimlib loop dm_multipath nfnetlink zram xfs crct10dif_ce ghash_ce sha2_ce sha256_arm64 sha1_ce virtio_blk virtio_console virtio_mmio dm_mirror dm_region_hash dm_log dm_mod fuse
[ 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
...



# cat reproducer.c
#include <stdio.h>
#include <sys/mman.h>
#include <linux/mman.h>
#include <errno.h>
#include <string.h>
#include <numaif.h>
#include <pthread.h>

#define SIZE_64KB (64 * 1024ul)

static void *thread_fn(void *arg)
{
         char *mem = arg;

         /* Let GUP go crazy on the page without grabbing a reference. */
         while (1) {
                 if (madvise(mem, SIZE_64KB, MADV_POPULATE_WRITE)) {
                         fprintf(stderr, "madvise() failed: %d\n", errno);
                 }
         }
}

int main(void)
{
         pthread_t thread;
         unsigned int i;
         char *mem;

         mem = mmap(0, SIZE_64KB, PROT_READ|PROT_WRITE,
                    MAP_ANON|MAP_PRIVATE|MAP_HUGETLB|MAP_HUGE_64KB, -1, 0);
         if (mem == MAP_FAILED) {
                 fprintf(stderr, "mmap() failed: %d\n", errno);
                 return -1;
         }

         memset(mem, 0, SIZE_64KB);

         pthread_create(&thread, NULL, thread_fn, mem);

         /* Migrate it back and forth between two nodes. */
         for (i = 0; ; i++) {
                 void *pages[1] = { mem, };
                 int nodes[1] = { i % 2, };
                 int status[1] = { 0 };

                 if (move_pages(0, 1, pages, nodes, status, MPOL_MF_MOVE_ALL))
                         fprintf(stderr, "move_pages failed: %d\n", errno);
                 if (status[0] != nodes[0])
                         printf("migration failed\n");
         }
         return 0;
}


-- 
Cheers,

David / dhildenb

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