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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jun 19, 2024 at 08:48:28AM -0700, 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"
> 
> 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"?

It's actually documented:

/*
 * There are some parts of the kernel which assume that PMD entries
 * are exactly HPAGE_PMD_ORDER.  Those should be fixed, but until then,
 * limit the maximum allocation order to PMD size.  I'm not aware of any
 * assumptions about maximum order if THP are disabled, but 8 seems like
 * a good order (that's 1MB if you're using 4kB pages)
 */
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
#define MAX_PAGECACHE_ORDER     HPAGE_PMD_ORDER
#else
#define MAX_PAGECACHE_ORDER     8
#endif

although I'm not even sure if we use it if CONFIG_TRANSPARENT_HUGEPAGE
is disabled.  All the machinery to split pages is gated by
CONFIG_TRANSPARENT_HUGEPAGE, so I think we end up completely ignoring
it.  I used to say "somebody should do the work to split out
CONFIG_LARGE_FOLIOS from CONFIG_TRANSPARENT_HUGEPAGE", but now I think
that nobody cares about the architectures that don't support it,
and it's not worth anybody's time pretending that we do.