6.6-stable review patch. If anyone has any objections, please let me know.
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From: Ryan Roberts ryan.roberts@arm.com
commit ec056cef76a525706601b32048f174f9bea72c7c upstream.
The THP machinery does not support order-1 folios because it requires meta data spanning the first 3 `struct page`s. So order-2 is the smallest large folio that we can safely create.
There was a theoretical bug whereby if ra->size was 2 or 3 pages (due to the device-specific bdi->ra_pages being set that way), we could end up with order = 1. Fix this by unconditionally checking if the preferred order is 1 and if so, set it to 0. Previously this was done in a few specific places, but with this refactoring it is done just once, unconditionally, at the end of the calculation.
This is a theoretical bug found during review of the code; I have no evidence to suggest this manifests in the real world (I expect all device-specific ra_pages values are much bigger than 3).
Link: https://lkml.kernel.org/r/20231201161045.3962614-1-ryan.roberts@arm.com Signed-off-by: Ryan Roberts ryan.roberts@arm.com Reviewed-by: Matthew Wilcox (Oracle) willy@infradead.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Hugh Dickins hughd@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/readahead.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-)
--- a/mm/readahead.c +++ b/mm/readahead.c @@ -514,16 +514,14 @@ void page_cache_ra_order(struct readahea unsigned int order = new_order;
/* Align with smaller pages if needed */ - if (index & ((1UL << order) - 1)) { + if (index & ((1UL << order) - 1)) order = __ffs(index); - if (order == 1) - order = 0; - } /* Don't allocate pages past EOF */ - while (index + (1UL << order) - 1 > limit) { - if (--order == 1) - order = 0; - } + while (index + (1UL << order) - 1 > limit) + order--; + /* THP machinery does not support order-1 */ + if (order == 1) + order = 0; err = ra_alloc_folio(ractl, index, mark, order, gfp); if (err) break;