Commit 83ffe150 authored by Ryan Roberts's avatar Ryan Roberts Committed by Kefeng Wang
Browse files

mm/readahead: do not allow order-1 folio 

mainline inclusion
from mainline-v6.8-rc1
commit ec056cef76a525706601b32048f174f9bea72c7c
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I98AW9
CVE: NA

-------------------------------------------------

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: default avatarRyan Roberts <ryan.roberts@arm.com>
Reviewed-by: default avatarMatthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
(cherry picked from commit ec056cef76a525706601b32048f174f9bea72c7c)
Signed-off-by: default avatarKefeng Wang <wangkefeng.wang@huawei.com>
parent 81088230

mm/readahead.c

+6 −8
Original line number Diff line number Diff line
@@ -514,16 +514,14 @@ 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)

		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;