Re: [PATCH 4/4] mm/shmem: Disable PMD-sized page cache if needed

[David Hildenbrand <david@redhat.com>]

On 25.06.24 11:06, Gavin Shan wrote:
> For shmem files, it's possible that PMD-sized page cache can't be
> supported by xarray. For example, 512MB page cache on ARM64 when
> the base page size is 64KB can't be supported by xarray. It leads
> to errors as the following messages indicate when this sort of xarray
> entry is split.
> 
> WARNING: CPU: 34 PID: 7578 at lib/xarray.c:1025 xas_split_alloc+0xf8/0x128
> Modules linked in: binfmt_misc 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: 34 PID: 7578 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 : ffff8000882af5f0
> x29: ffff8000882af5f0 x28: ffff8000882af650 x27: ffff8000882af768
> x26: 0000000000000cc0 x25: 000000000000000d x24: ffff00010625b858
> x23: ffff8000882af650 x22: ffffffdfc0900000 x21: 0000000000000000
> x20: 0000000000000000 x19: ffffffdfc0900000 x18: 0000000000000000
> x17: 0000000000000000 x16: 0000018000000000 x15: 52f8004000000000
> x14: 0000e00000000000 x13: 0000000000002000 x12: 0000000000000020
> x11: 52f8000000000000 x10: 52f8e1c0ffff6000 x9 : ffffbeb9619a681c
> x8 : 0000000000000003 x7 : 0000000000000000 x6 : ffff00010b02ddb0
> x5 : ffffbeb96395e378 x4 : 0000000000000000 x3 : 0000000000000cc0
> 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
>   shmem_undo_range+0x2bc/0x6a8
>   shmem_fallocate+0x134/0x430
>   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 disabling PMD-sized page cache when HPAGE_PMD_ORDER is
> larger than MAX_PAGECACHE_ORDER.
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>   mm/shmem.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index a8b181a63402..5453875e3810 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -541,8 +541,9 @@ static bool shmem_confirm_swap(struct address_space *mapping,
>   
>   static int shmem_huge __read_mostly = SHMEM_HUGE_NEVER;
>   
> -bool shmem_is_huge(struct inode *inode, pgoff_t index, bool shmem_huge_force,
> -		   struct mm_struct *mm, unsigned long vm_flags)
> +static bool __shmem_is_huge(struct inode *inode, pgoff_t index,
> +			    bool shmem_huge_force, struct mm_struct *mm,
> +			    unsigned long vm_flags)
>   {
>   	loff_t i_size;
>   
> @@ -573,6 +574,16 @@ bool shmem_is_huge(struct inode *inode, pgoff_t index, bool shmem_huge_force,
>   	}
>   }
>   
> +bool shmem_is_huge(struct inode *inode, pgoff_t index,
> +		   bool shmem_huge_force, struct mm_struct *mm,
> +		   unsigned long vm_flags)
> +{
> +	if (!__shmem_is_huge(inode, index, shmem_huge_force, mm, vm_flags))
> +		return false;
> +
> +	return HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER;

Why not check for that upfront?

> +}
> +
>   #if defined(CONFIG_SYSFS)
>   static int shmem_parse_huge(const char *str)
>   {

This should make __thp_vma_allowable_orders() happy for shmem, and consequently, also khugepaged IIRC.

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


@Ryan,

should we do something like the following on top? The use of PUD_ORDER for ordinary pagecache is
wrong. Really only DAX is special and can support that in its own weird ways.

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 2aa986a5cd1b..ac63233fed6c 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -72,14 +72,25 @@ extern struct kobj_attribute shmem_enabled_attr;
  #define THP_ORDERS_ALL_ANON    ((BIT(PMD_ORDER + 1) - 1) & ~(BIT(0) | BIT(1)))
  
  /*
- * Mask of all large folio orders supported for file THP.
+ * Mask of all large folio orders supported for FSDAX THP.
   */
-#define THP_ORDERS_ALL_FILE    (BIT(PMD_ORDER) | BIT(PUD_ORDER))
+#define THP_ORDERS_ALL_DAX     (BIT(PMD_ORDER) | BIT(PUD_ORDER))
+
+
+/*
+ * Mask of all large folio orders supported for ordinary pagecache (file/shmem)
+ * THP.
+ */
+#if PMD_ORDER <= MAX_PAGECACHE_ORDER
+#define THP_ORDERS_ALL_FILE    0
+#else
+#define THP_ORDERS_ALL_FILE    (BIT(PMD_ORDER))
+#endif
  
  /*
   * Mask of all large folio orders supported for THP.
   */
-#define THP_ORDERS_ALL         (THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_FILE)
+#define THP_ORDERS_ALL         (THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_FILE )
  
  #define TVA_SMAPS              (1 << 0)        /* Will be used for procfs */
  #define TVA_IN_PF              (1 << 1)        /* Page fault handler */
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 89932fd0f62e..95d4a2edae39 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -88,9 +88,15 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
         bool smaps = tva_flags & TVA_SMAPS;
         bool in_pf = tva_flags & TVA_IN_PF;
         bool enforce_sysfs = tva_flags & TVA_ENFORCE_SYSFS;
+
         /* Check the intersection of requested and supported orders. */
-       orders &= vma_is_anonymous(vma) ?
-                       THP_ORDERS_ALL_ANON : THP_ORDERS_ALL_FILE;
+       if (vma_is_anonymous(vma))
+               orders &= THP_ORDERS_ALL_ANON;
+       else if (vma_is_dax(vma))
+               orders &= THP_ORDERS_ALL_DAX;
+       else
+               orders &= THP_ORDERS_ALL_FILE;
+
         if (!orders)
                 return 0;
  



-- 
Cheers,

David / dhildenb