CMA allocation should be guaranteed to succeed by definition, but,
unfortunately, it would be failed sometimes. It is hard to track down
the problem, because it is related to page reference manipulation and we
don't have any facility to analyze it.
This patch adds tracepoints to track down page reference manipulation.
With it, we can find exact reason of failure and can fix the problem.
Following is an example of tracepoint output. (note: this example is
stale version that printing flags as the number. Recent version will
print it as human readable string.)
<...>-9018 [004] 92.678375: page_ref_set: pfn=0x17ac9 flags=0x0 count=1 mapcount=0 mapping=(nil) mt=4 val=1
<...>-9018 [004] 92.678378: kernel_stack:
=> get_page_from_freelist (ffffffff81176659)
=> __alloc_pages_nodemask (ffffffff81176d22)
=> alloc_pages_vma (ffffffff811bf675)
=> handle_mm_fault (ffffffff8119e693)
=> __do_page_fault (ffffffff810631ea)
=> trace_do_page_fault (ffffffff81063543)
=> do_async_page_fault (ffffffff8105c40a)
=> async_page_fault (ffffffff817581d8)
[snip]
<...>-9018 [004] 92.678379: page_ref_mod: pfn=0x17ac9 flags=0x40048 count=2 mapcount=1 mapping=0xffff880015a78dc1 mt=4 val=1
[snip]
...
...
<...>-9131 [001] 93.174468: test_pages_isolated: start_pfn=0x17800 end_pfn=0x17c00 fin_pfn=0x17ac9 ret=fail
[snip]
<...>-9018 [004] 93.174843: page_ref_mod_and_test: pfn=0x17ac9 flags=0x40068 count=0 mapcount=0 mapping=0xffff880015a78dc1 mt=4 val=-1 ret=1
=> release_pages (ffffffff8117c9e4)
=> free_pages_and_swap_cache (ffffffff811b0697)
=> tlb_flush_mmu_free (ffffffff81199616)
=> tlb_finish_mmu (ffffffff8119a62c)
=> exit_mmap (ffffffff811a53f7)
=> mmput (ffffffff81073f47)
=> do_exit (ffffffff810794e9)
=> do_group_exit (ffffffff81079def)
=> SyS_exit_group (ffffffff81079e74)
=> entry_SYSCALL_64_fastpath (ffffffff817560b6)
This output shows that problem comes from exit path. In exit path, to
improve performance, pages are not freed immediately. They are gathered
and processed by batch. During this process, migration cannot be
possible and CMA allocation is failed. This problem is hard to find
without this page reference tracepoint facility.
Enabling this feature bloat kernel text 30 KB in my configuration.
text data bss dec hex filename
12127327 2243616 1507328 15878271 f2487f vmlinux_disabled
12157208 2258880 1507328 15923416 f2f8d8 vmlinux_enabled
Note that, due to header file dependency problem between mm.h and
tracepoint.h, this feature has to open code the static key functions for
tracepoints. Proposed by Steven Rostedt in following link.
https://lkml.org/lkml/2015/12/9/699
[arnd@arndb.de: crypto/async_pq: use __free_page() instead of put_page()]
[iamjoonsoo.kim@lge.com: fix build failure for xtensa]
[akpm@linux-foundation.org: tweak Kconfig text, per Vlastimil]
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
The success of CMA allocation largely depends on the success of
migration and key factor of it is page reference count. Until now, page
reference is manipulated by direct calling atomic functions so we cannot
follow up who and where manipulate it. Then, it is hard to find actual
reason of CMA allocation failure. CMA allocation should be guaranteed
to succeed so finding offending place is really important.
In this patch, call sites where page reference is manipulated are
converted to introduced wrapper function. This is preparation step to
add tracepoint to each page reference manipulation function. With this
facility, we can easily find reason of CMA allocation failure. There is
no functional change in this patch.
In addition, this patch also converts reference read sites. It will
help a second step that renames page._count to something else and
prevents later attempt to direct access to it (Suggested by Andrew).
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Joonsoo Kim