| Message ID | 20231201161045.3962614-1-ryan.roberts@arm.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v1] mm/readahead: Do not allow order-1 folio | expand |
On Fri, Dec 01, 2023 at 04:10:45PM +0000, Ryan Roberts wrote: > 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). > > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> It's better code anyway! Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
diff --git a/mm/readahead.c b/mm/readahead.c index 6925e6959fd3..23620c57c122 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -511,16 +511,14 @@ void page_cache_ra_order(struct readahead_control *ractl, 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;
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). Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> --- mm/readahead.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) -- 2.25.1