Re: Endless calls to xas_split_alloc() due to corrupted xarray entry

[David Hildenbrand <david@redhat.com>]

On 19.06.24 17:48, Linus Torvalds wrote:
> On Wed, 19 Jun 2024 at 07:31, Matthew Wilcox <willy@infradead.org> wrote:
>>
>> Actually, it's 11.  We can't split an order-12 folio because we'd have
>> to allocate two levels of radix tree, and I decided that was too much
>> work.  Also, I didn't know that ARM used order-13 PMD size at the time.
>>
>> I think this is the best fix (modulo s/12/11/).
> 
> Can we use some more descriptive thing than the magic constant 11 that
> is clearly very subtle.
> 
> Is it "XA_CHUNK_SHIFT * 2 - 1"

That's my best guess as well :)

> 
> IOW, something like
> 
>     #define MAX_XAS_ORDER (XA_CHUNK_SHIFT * 2 - 1)
>     #define MAX_PAGECACHE_ORDER min(HPAGE_PMD_ORDER,12)
> 
> except for the non-TRANSPARENT_HUGEPAGE case where it currently does
> 
>    #define MAX_PAGECACHE_ORDER    8
> 
> and I assume that "8" is just "random round value, smaller than 11"?

Yes, that matches my understanding.

Maybe to be safe for !THP as well, something ike:

+++ b/include/linux/pagemap.h
@@ -354,11 +354,18 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
   * a good order (that's 1MB if you're using 4kB pages)
   */
  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-#define MAX_PAGECACHE_ORDER	HPAGE_PMD_ORDER
+#define WANTED_MAX_PAGECACHE_ORDER	HPAGE_PMD_ORDER
  #else
-#define MAX_PAGECACHE_ORDER	8
+#define WANTED_MAX_PAGECACHE_ORDER	8
  #endif
  
+/*
+ * xas_split_alloc() does not support arbitrary orders yet. This implies no
+ * 512MB THP on arm64 with 64k.
+ */
+#define MAX_XAS_ORDER		(XA_CHUNK_SHIFT * 2 - 1)
+#define MAX_PAGECACHE_ORDER	min(MAX_XAS_ORDER, WANTED_MAX_PAGECACHE_ORDER)
+
  /**
   * mapping_set_large_folios() - Indicate the file supports large folios.
   * @mapping: The file.
-- 
2.45.2


@Gavin, do you have capacity to test+prepare an official patch? Also,
please double-check whether shmem must be fenced as well (very likely).

This implies no PMD-sized THPs in the pagecache/shmem on arm64 with 64k.
Could be worse, because as Willy said, they are rather rare and extremely
unpredictable.

-- 
Cheers,

David / dhildenb