Russ Anderson
2008-07-18T20:35:14Z

[PATCH 0/2] Migrate data off physical pages with corrected memory errors (Version 7)

Version 7 changes: resubmitting for 2.6.27
page.discard.v7:
- Add pagemask macros in page-flags.h (per Christoph
Lameter's request).
- refreshed for linux-next

cpe.migrate.v7:
- refreshed for linux-next

page.cleanup
- Accepted by Linus.

Version 6 changes:
page.cleanup.v6:
- Fix cut-n-paste comment (per Linus Torvalds's request).

page.discard.v6:
- Fixed a problem where a page failed to migrate would end
up with an extra reference count (per Christoph Lameter's
request).
- Fixed comments (per Christoph Lameter's request).
- Moved totalbad_pages definition from mm/migrate.c to
arch/ia64/kernel/mca.c (per Christoph Lameter's request).

cpe.migrate.v6:
- Move totalbad_pages from mm/migrate.c to ia64/kernel/mca.c.
- Replace tab with space (per Christoph Lameter's request).

Version 5 changes:
page.cleanup.v5:
- Change names to reflect the use and add comments to explain
the meaning. (per Linus Torvalds's request).

Version 4 changes:
page.discard.v4:
- Remove the hot path checks in lru_cache_add() and
lru_cache_add_active(). Avoid moving the bad page to
the LRU in unmap_and_move() and putback_lru_pages().
(per Linus Torvalds's request).

cpe.migrate.v4
- More code style cleanup (per Andrew Morton's request).
- Removed locking when calling the migration code
(per Christoph Lameter's request).
- If the page fails to migrate, clear the PG_memerror flag.
This avoids a page with PG_memerror on the free list.

Version 3 changes:
page.cleanup.v3:
- Put PAGE_FLAGS definitions back in page-flags.h
(per Christoph Lameter's request).

cpe.migrate.v3
- Use putback_lru_pages() when returning an individual page
(per Christoph Lameter's request).
- Code style cleanup
(per Pekka Enberg's request).
- Use strict_strtoul()
(per Pekka Enberg's request).
- Added locking
(per Pekka Enberg's request).
- Use /sys/kernel/ instead of /proc
(per Pekka Enberg's request).

Version 2 changes:

Broke the page.discard patch into two patches, per request by
Christoph Lameter.

page.cleanup.v2:
- minor clean-up of page flags in page_alloc.c.

page.discard.v2:
- Updated for recent page flag clean-up.
- Removed the change to the sysinfo struct.

cpe.migrate.v2
- Added /proc/badram interface to print page discard
information and to free bad pages.

Purpose:

Physical memory with corrected errors may decay over time into
uncorrectable errors. The purpose of this patch is to move the
data off pages with correctable memory errors before the memory
goes bad.

The patches:

[1/3] page.cleanup.v6: Minor clean-up of page flags in mm/page_alloc.c

Minor source code clean-up of page flags in mm/page_alloc.c.
The cleanup makes it easier for the next patch to add PG_memerror.

[2/3] page.discard.v6: Avoid putting a bad page back on the LRU.

page.discard.v6 are the arch independent changes. It adds a new
page flag (PG_memerror) to mark the page as bad and avoids putting
the page back on the LRU after migrating the data to a new page.
The reference count on the bad page is not decremented to zero to
avoid it being reallocated. PG_memerror is only defined if
CONFIG_PAGEFLAGS_EXTENDED is defined.

[3/3] cpe.migrate.v6: Call migration code on correctable errors

cpe.migrate.v6 are the IA64 specific changes. It connects the CPE
handler to the page migration code. It is implemented as a kernel
loadable module, similar to the mca recovery code (mca_recovery.ko),
so that it can be removed to turn off the feature. Create
/sys/kernel/badram to print page discard information and to free
bad pages.

Comments:

There is always an issue of how agressive the code should be on
migrating pages. Should it migrate on the first correctable error,
or wait for some threshold? Reasonable people may disagree on the
threshold and the "right" answer may be hardware specific. The
decision making is confined to the cpe_migrate.c code and can be
built as a kernel loadable module. It is currently set to migrate
on the first correctable error.

Only pages that can be isolated on the LRU are migrated. Other
pages, such as compound pages, are not migrated. That functionality
could be a future enhancement.

/sys/kernel/badram is a way of displaying information about the bad
memory and freeing the bad pages. A userspace program (or sysadmin)
could determine if a discarded page needs to be freed.

Sample output:

linux> insmod cpe_migrate.ko
linux> cat /sys/kernel/badram // This shows no discarded memory
Bad RAM: 0 kB, 0 pages marked bad
List of bad physical pages

linux> ./errsingle -c 6 -s 1 // Inject correctable errors on
// six pages.
linux> cat /sys/kernel/badram
Bad RAM: 384 kB, 6 pages marked bad
List of bad physical pages
0x06048e10000 0x06870c40000 0x06870c20000 0x06870c10000 0x06007f00000
0x06042070000

linux> echo 0x06870c20000 > /sys/kernel/badram // Free one of the pages

linux> cat /sys/kernel/badram // Five pages remain on the list
Bad RAM: 320 kB, 5 pages marked bad
List of bad physical pages
0x06048e10000 0x06870c40000 0x06870c10000 0x06007f00000 0x06042070000

linux> echo 0 > /sys/kernel/badram // Free all the bad pages
linux> cat /sys/kernel/badram // All the pages are freed
Bad RAM: 0 kB, 0 pages marked bad
List of bad physical pages

Flow of the code description (while testing on IA64):

1) A user level application test program allocates memory and
passes the virtual address of the memory to the error injection
driver.

2) The error injection driver converts the virtual address to
physical, functions the Altix hardware to modify the ECC for the
physical page, creating a correctable error, and returns to the
user application.

3) The user application reads the memory.

4) The Altix hardware detects the correctable error and interrupts
prom. SAL builds a CPU error record, then sends a CPE
interrupt to linux.

5) The linux CPE handler calls the cpe_migrate module (if installed).

6) cpe_migrate parses the physical address from the CPE record and
adds the address to the migrate list (if not already on the list)
and schedules the worker thread (cpe_enable_work).

7) cpe_enable_work calls ia64_mca_cpe_move_page.

8) ia64_mca_cpe_move_page validates the physical address, converts
to page, sets PG_memerror flag and calls the migration code
(migrate_prep(), isolate_lru_page(), and migrate_pages(). If the
page migrates successfully, the bad page is added to badpagelist.

9) Because PG_memerror is set, the bad page is not added back on the LRU
by avoiding calls to move_to_lru(). Avoiding move_to_lru() prevents
the page count from being decremented to zero.

10) If the page fails to migrate, PG_memerror is cleared and the page
returned to the LRU. If another correctable error occurs on the
page another attempt will be made to migrate it.