On 2020-06-29 15:51, Matthew Wilcox wrote:
On Mon, Jun 29, 2020 at 03:38:13PM -0700, John Hubbard wrote:
On 2020-06-29 08:19, Matthew Wilcox (Oracle) wrote:
Tail page flags contain very little useful information. Print the head
page's flags instead (even though PageHead is a little misleading).
You are right about the tail page. And the raw output provides the tail
page flags, in case someone *really* needs to dig into tail page problems,
so that's all good.
However, I just gave this a spin, and seeing the "|head" in the list for
my "tail page: dump_page test" is also slightly misleading for me, too.
We could also do ...
@@ -48,6 +48,8 @@ void __dump_page(struct page *page, const char *reason)
struct address_space *mapping;
bool page_poisoned = PagePoisoned(page);
bool compound = PageCompound(page);
+ unsigned long flags = page->flags;
+
/*
* Accessing the pageblock without the zone lock. It could change to
* "isolate" again in the meantime, but since we are just dumping the
@@ -165,7 +162,9 @@ void __dump_page(struct page *page, const char *reason)
out_mapping:
BUILD_BUG_ON(ARRAY_SIZE(pageflag_names) != __NR_PAGEFLAGS + 1);
- pr_warn("%sflags: %#lx(%pGp)%s\n", type, head->flags, &head->flags,
+ if (head != page)
+ flags = head->flags & ~PG_head;
The above should be:
flags = head->flags & ~(1UL << PG_head);
+ pr_warn("%sflags: %#lx(%pGp)%s\n", type, flags, &flags,
page_cma ? " CMA" : "");
hex_only:
...so with that fix, along with your line break approach in the other thread,
a tail page dump of a FOLL_PIN page looks like this:
[ 38.027987] page:00000000abaef9ae refcount:513 mapcount:1 mapping:0000000000000000 index:0x11
[ 38.035633] head:00000000675be53c order:9 compound_mapcount:1 compound_pincount:512
[ 38.049155] anon flags: 0x17ffe000000000e(referenced|uptodate|dirty)
[ 38.055465] raw: 017ffe0000000000 ffffea0020dd0001 ffffea0020dd0448 dead000000000400
[ 38.062319] raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
[ 38.069183] head: 017ffe000001000e ffffffff83649ca0 ffffea0020dd8008 ffff88888e0b6641
[ 38.076141] head: 0000000000000000 0000000000000000 00000201ffffffff 0000000000000000
[ 38.083102] page dumped because: gup_benchmark: tail page: dump_page test
So, good. However, I feel that the "head " prefix approach is slightly
preferable, because it's doing less processing (the more code one
adds to little-exercised debug paths, the more likely the debugging has
bugs) and is instead just printing out what it sees directly. And it seems a little
odd to remove the PG_head bit from the output.
The "head " prefix approach looks like this:
[ 38.027987] page:00000000abaef9ae refcount:513 mapcount:1 mapping:0000000000000000 index:0x11
[ 38.035633] head:00000000675be53c order:9 compound_mapcount:1 compound_pincount:512
[ 38.049155] head anon flags: 0x17ffe000000000e(referenced|uptodate|dirty|head)
[ 38.055465] raw: 017ffe0000000000 ffffea0020dd0001 ffffea0020dd0448 dead000000000400
[ 38.062319] raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
[ 38.069183] head: 017ffe000001000e ffffffff83649ca0 ffffea0020dd8008 ffff88888e0b6641
[ 38.076141] head: 0000000000000000 0000000000000000 00000201ffffffff 0000000000000000
[ 38.083102] page dumped because: gup_benchmark: tail page: dump_page test
thanks,
--
John Hubbard
NVIDIA
