Re: [PATCH 2/4] mm/filemap: Skip to allocate PMD-sized folios if needed

[David Hildenbrand <david@redhat.com>]

On 25.06.24 11:06, Gavin Shan wrote:
> On ARM64, HPAGE_PMD_ORDER is 13 when the base page size is 64KB. The
> PMD-sized page cache can't be supported by xarray as the following
> error messages indicate.
> 
> ------------[ cut here ]------------
> WARNING: CPU: 35 PID: 7484 at lib/xarray.c:1025 xas_split_alloc+0xf8/0x128
> 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    \
> ip_set rfkill nf_tables nfnetlink vfat fat virtio_balloon drm      \
> fuse xfs libcrc32c crct10dif_ce ghash_ce sha2_ce sha256_arm64      \
> sha1_ce virtio_net net_failover virtio_console virtio_blk failover \
> dimlib virtio_mmio
> CPU: 35 PID: 7484 Comm: test Kdump: loaded Tainted: G W 6.10.0-rc5-gavin+ #9
> Hardware name: QEMU KVM Virtual Machine, BIOS edk2-20240524-1.el9 05/24/2024
> pstate: 83400005 (Nzcv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
> pc : xas_split_alloc+0xf8/0x128
> lr : split_huge_page_to_list_to_order+0x1c4/0x720
> sp : ffff800087a4f6c0
> x29: ffff800087a4f6c0 x28: ffff800087a4f720 x27: 000000001fffffff
> x26: 0000000000000c40 x25: 000000000000000d x24: ffff00010625b858
> x23: ffff800087a4f720 x22: ffffffdfc0780000 x21: 0000000000000000
> x20: 0000000000000000 x19: ffffffdfc0780000 x18: 000000001ff40000
> x17: 00000000ffffffff x16: 0000018000000000 x15: 51ec004000000000
> x14: 0000e00000000000 x13: 0000000000002000 x12: 0000000000000020
> x11: 51ec000000000000 x10: 51ece1c0ffff8000 x9 : ffffbeb961a44d28
> x8 : 0000000000000003 x7 : ffffffdfc0456420 x6 : ffff0000e1aa6eb8
> x5 : 20bf08b4fe778fca x4 : ffffffdfc0456420 x3 : 0000000000000c40
> x2 : 000000000000000d x1 : 000000000000000c x0 : 0000000000000000
> Call trace:
>   xas_split_alloc+0xf8/0x128
>   split_huge_page_to_list_to_order+0x1c4/0x720
>   truncate_inode_partial_folio+0xdc/0x160
>   truncate_inode_pages_range+0x1b4/0x4a8
>   truncate_pagecache_range+0x84/0xa0
>   xfs_flush_unmap_range+0x70/0x90 [xfs]
>   xfs_file_fallocate+0xfc/0x4d8 [xfs]
>   vfs_fallocate+0x124/0x2e8
>   ksys_fallocate+0x4c/0xa0
>   __arm64_sys_fallocate+0x24/0x38
>   invoke_syscall.constprop.0+0x7c/0xd8
>   do_el0_svc+0xb4/0xd0
>   el0_svc+0x44/0x1d8
>   el0t_64_sync_handler+0x134/0x150
>   el0t_64_sync+0x17c/0x180
> 
> Fix it by skipping to allocate PMD-sized page cache when its size
> is larger than MAX_PAGECACHE_ORDER. For this specific case, we will
> fall to regular path where the readahead window is determined by BDI's
> sysfs file (read_ahead_kb).
> 
> Fixes: 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings")
> Cc: stable@kernel.org # v5.18+
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>   mm/filemap.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 876cc64aadd7..b306861d9d36 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3124,7 +3124,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>   
>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>   	/* Use the readahead code, even if readahead is disabled */
> -	if (vm_flags & VM_HUGEPAGE) {
> +	if ((vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) {
>   		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
>   		ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
>   		ra->size = HPAGE_PMD_NR;

As discussed, one option is for using min(HPAGE_PMD_ORDER, 
MAX_PAGECACHE_ORDER) here, but it also doesn't quite result in the 
expected performance results on arm64 with 64k.

This code dates back to PMD-THP times, so we'll leave it like that for now.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb