Re: [PATCH 2/2] mm: introduce PAGE_FLAGS() to make output of page flags better
From: David Hildenbrand <hidden>
Date: 2021-01-15 08:39:18
On 15.01.21 07:13, Yafang Shao wrote:
There're totally __NR_PAGEFLAGS page flags, but the type of page->flags is unsigned long, that makes the value of page->flags a little misleading when it is printed to the user. We'd better print the real pages flags, instead of the whole 64bits including the random values in the useless high bits.
No, these are *not* random values. They include the nid, zid, and section_nr - which are helpful to have at hand when debugging, or detecting that something might be messed up there.
There're two choices to achieve that, one of which is clear the useless
Again, not useless.
high bits when we initlize the page->flags, the other is don't print the high bits when it is showed to the user. The latter one is better because it is in the slow path and the performance won't be impacted. Before that change, the output is, [ 8846.517809] INFO: Slab 0x00000000f42a2c60 objects=33 used=3 fp=0x0000000060d32ca8 flags=0x17ffffc0010200(slab|head) After that change, the output is, [ 8843.757770] INFO: Slab 0x00000000f0e98335 objects=33 used=3 fp=0x00000000b643c7d8 flags=0x10200(slab|head)
Nack to the current approach. If you're going to strip this information, you should expose it differently. E.g., printing page_zonenum() or page_to_nid(). But still, then we might lose valuable information of bits stored in there that shouldn't have been set.
quoted hunk ↗ jump to hunk
Signed-off-by: Yafang Shao <redacted> --- include/linux/page-flags.h | 2 ++ mm/slub.c | 2 +- 2 files changed, 3 insertions(+), 1 deletion(-)diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index ec5d029..db5c017 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h@@ -175,6 +175,8 @@ enum pageflags { PG_reported = PG_uptodate, }; +#define PAGE_FLAGS(flags) (flags & ((1 << __NR_PAGEFLAGS) - 1)) + #ifndef __GENERATING_BOUNDS_H struct page; /* forward declaration */diff --git a/mm/slub.c b/mm/slub.c index 901a45a..a93a03c 100644 --- a/mm/slub.c +++ b/mm/slub.c@@ -643,7 +643,7 @@ static void print_page_info(struct page *page) { pr_err("INFO: Slab 0x%p objects=%u used=%u fp=0x%p flags=%#lx(%pGp)\n", page, page->objects, page->inuse, page->freelist, - page->flags, &page->flags); + PAGE_FLAGS(page->flags), &page->flags); }
-- Thanks, David / dhildenb