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

[Gavin Shan <gshan@redhat.com>]

On 6/20/24 5:58 AM, David Hildenbrand wrote:
> 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.

Thanks David. I'm checking if shmem needs the similar limitation and test patches.
I will post them for review once they're ready.

Thanks,
Gavin