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Subject[PATCH 0/2] ia64: Migrate data off physical pages with correctable errors
	Migrate data off physical pages with corrected memory errors

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:

[1/2] page.discard: Avoid putting a bad page back on the LRU.

page.discard are the arch independent changes. It adds a new
page flag (PG_memerror) to mark the page as bad and prevent it
from being put back on the LRU. PG_memerror is bit 32 and only
defined on 64 bit architectures. It adds "BadPages:" output to
/proc/meminfo on 64 bit architectures.

[2/2] cpe.migrate: Call migration code on correctable errors

cpe.migrate 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. It exports
three symbols (migrate_prep, isolate_lru_page, and migrate_pages).

Comments:

Since page flags are a precious commodity on 32 bit architectures,
the choice was made to implement PG_memerror only on 64 bit
architectures, in the upper 32 bits. If there is interest in
this feature on 32 bit, it only requires defining PG_memerror
in one of the lower 32 page flage bits and removing the BITS_PER_LONG
checks.

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. 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.

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
due to checks in lru_cache_add() and lru_cache_add_active().

--
Russ Anderson, OS RAS/Partitioning Project Lead
SGI - Silicon Graphics Inc rja@sgi.com


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