On Wed, Jun 19, 2024 at 08:48:28AM -0700, Linus Torvalds wrote: > On Wed, 19 Jun 2024 at 07:31, Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.
