Re: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v5

Wu Fengguang
2009-06-04T09:15:35Z

On Thu, Jun 04, 2009 at 01:13:46PM +0800, Andi Kleen wrote:
> On Thu, Jun 04, 2009 at 11:24:41AM +0800, Wu Fengguang wrote:
> > On Thu, Jun 04, 2009 at 02:46:47AM +0800, Andi Kleen wrote:
> >
> > [snip]
> >
> > This patch is full of this style error (the old version didn't have
> > this problem though):
>
> I don't see that here. At least nothing new compared to old.
>
> >
> > ERROR: code indent should use tabs where possible
>
> It's checkpath clean for me, except for a few > 80 lines on printks,
> one list_for_each_entry_safe (which I think checkpatch is wrong on) and
> the meminfo comma error which I also think checkpath.pl is wrong on too.

OK, that's fine. Maybe some email server expanded tabs in between.
I wonder whether its the send side or the receive side, ie. whether
it affected more people..

> > > + page_cache_release(p);
> > > +
> > > + /*
> > > + * Now truncate the page in the page cache. This is really
> > > + * more like a "temporary hole punch"
> > > + * Don't do this for block devices when someone else
> > > + * has a reference, because it could be file system metadata
> > > + * and that's not safe to truncate.
> > > + */
> > > + mapping = page_mapping(p);
> > > + if (mapping && S_ISBLK(mapping->host->i_mode) && page_count(p) > 1) {
> >
> > Shall use (page_count > 2) to count for the page cache reference.
>
> I think the page cache reference got dropped in
>
> if (!isolate_lru_page(p))
> page_cache_release(p);
>
> So it should be only one if there are no other users

Ah right!

> > Or can we base the test on busy buffers instead of page count? Nick?
>
> At least the S_ISBLK test is the best one I came up with. I'm not
> saying it's the absolutely best.

Yes I agree with the S_ISBLK test and was questioning the page count
test. btw, one exception to the S_ISBLK test is btrfs, which does not
use blockdev for metadata.

> > > + SetPageError(p);
> > > + /* TBD: print more information about the file. */
> > > + if (mapping) {
> > > + /*
> > > + * IO error will be reported by write(), fsync(), etc.
> > > + * who check the mapping.
> >
> > btw, here are some side notes on EIO.
> >
> > close() *may* also report it. NFS will sync file on close.
>
> I think the comment is already too verbose, sure there are other
> details too that it doesn't describe. It's not trying to be a
> full reference on linux error reporting. So I prefer to not
> add more cases.

Yes, I was not asking for expanding the long comment :-)

> > > + * at the wrong time.
> > > + *
> > > + * So right now we assume that the application DTRT on
> >
> > DTRT = do the return value test?
>
> Do The Right Thing

OK.

> > > +};
> > > +
> > > +static void action_result(unsigned long pfn, char *msg, int ret)
> >
> > rename 'ret' to 'action'?
>
> But's not an action (as in a page state handler), it's a return value?
> (RECOVERED, FAILED etc.) I can name it result.

Ah yes, it's return code.

> > > + * need this to decide if we should kill or just drop the page.
> > > + */
> > > + mapping = page_mapping(p);
> > > + if (!PageDirty(p) && !PageAnon(p) && !PageSwapBacked(p) &&
> >
> > !PageAnon(p) could be removed: the below non-zero mapping check will
> > do the work implicitly.
>
> You mean !page_mapped? Ok.

I mean to do
mapping = page_mapping(p);
if (!PageDirty(p) && !PageSwapBacked(p) &&
mapping && mapping_cap_account_dirty(mapping)) {

Because for anonymous pages, page_mapping == NULL.

> > > + kill = 0;
> > > + printk(KERN_INFO
> > > + "MCE %#lx: corrupted page was clean: dropped without side effects\n",
> > > + pfn);
> > > + ttu |= TTU_IGNORE_HWPOISON;
> >
> > Why not put the two assignment lines together? :)
>
> Ok. But that was your patch @)

Yes, so is the above one ;)

> > > + * Try a few times (RED-PEN better strategy?)
> > > + */
> > > + for (i = 0; i < N_UNMAP_TRIES; i++) {
> > > + ret = try_to_unmap(p, ttu);
> > > + if (ret == SWAP_SUCCESS)
> > > + break;
> > > + pr_debug("MCE %#lx: try_to_unmap retry needed %d\n", pfn, ret);
> >
> > Can we make it a printk? This is a serious accident.
>
> I think it can actually happen due to races, e.g. when a remap
> is currently in process.

When it happened, the page may not be isolated from pte and page cache,
and thus very likely to damage the system. So add a warning when failed?

--- sound-2.6.orig/mm/memory-failure.c
+++ sound-2.6/mm/memory-failure.c
@@ -660,6 +660,10 @@ static void hwpoison_user_mappings(struc
break;
pr_debug("MCE %#lx: try_to_unmap retry needed %d\n", pfn, ret);
}
+ if (ret != SWAP_SUCCESS)
+ printk(KERN_ERR
+ "MCE %#lx: failed to unmap page (mapcount=%d)!\n",
+ pfn, page_mapcount(p));

/*
* Now that the dirty bit has been propagated to the

> > > + */
> > > + hwpoison_user_mappings(p, pfn, trapno);
> > > +
> > > + /*
> > > + * Torn down by someone else?
> > > + */
> > > + if (PageLRU(p) && !PageSwapCache(p) && p->mapping == NULL) {
> > > + action_result(pfn, "already unmapped LRU", IGNORED);
> >
> > "NULL mapping LRU" or "already truncated page"?
> > At least page_mapped != page_mapping.
>
> It's "already truncated" now.

Thanks.

> > > @@ -1311,6 +1311,20 @@
> > > .mode = 0644,
> > > .proc_handler = &scan_unevictable_handler,
> > > },
> > > +#ifdef CONFIG_MEMORY_FAILURE
> > > + {
> > > + .ctl_name = CTL_UNNUMBERED,
> > > + .procname = "memory_failure_early_kill",
> > > + .data = &sysctl_memory_failure_early_kill,
> > > + .maxlen = sizeof(vm_highmem_is_dirtyable),
> >
> > s/vm_highmem_is_dirtyable/sysctl_memory_failure_early_kill/
>
> Fixed thanks.
>
> > > * Documentation/sysctl/ctl_unnumbered.txt
> > > Index: linux/fs/proc/meminfo.c
> > > ===================================================================
> > > --- linux.orig/fs/proc/meminfo.c 2009-06-03 19:37:38.000000000 +0200
> > > +++ linux/fs/proc/meminfo.c 2009-06-03 20:13:43.000000000 +0200
> > > @@ -95,7 +95,11 @@
> > > "Committed_AS: %8lu kB\n"
> > > "VmallocTotal: %8lu kB\n"
> > > "VmallocUsed: %8lu kB\n"
> > > - "VmallocChunk: %8lu kB\n",
> > > + "VmallocChunk: %8lu kB\n"
> > > +#ifdef CONFIG_MEMORY_FAILURE
> > > + "BadPages: %8lu kB\n"
> >
> > "HWPoison:" or something like that?
> > People is more likely to misinterpret "BadPages".
>
> I'll name it HardwareCorrupted. That makes it too long, but it's hopefully
> clearer.

That's OK. Maybe we need a standalone alignment patch for /proc/meminfo ;-)

> > > vmi.used >> 10,
> > > vmi.largest_chunk >> 10
> > > +#ifdef CONFIG_MEMORY_FAILURE
> > > + ,atomic_long_read(&mce_bad_pages) << (PAGE_SHIFT - 10)
> >
> > ERROR: space required after that ','
>
> That's one of the cases where checkpatch.pl is stupid. The lone comma
> with a space looks absolutely ridiculous to me. I refuse to do ridiculous
> things things just for checkpatch.pl deficiencies.

OK.

> > > Enable the KSM kernel module to allow page sharing of equal pages
> > > among different tasks.
> > >
> > > +config MEMORY_FAILURE
> > > + bool
> > > +
> >
> > Do we have code to automatically enable/disable CONFIG_MEMORY_FAILURE
> > based on hardware capability?
>
> Yes the architecture can enable it. There's also another patch
> which always enables it for testing.

OK.

> > > +
> > > +Control how to kill processes when uncorrected memory error (typically
> > > +a 2bit error in a memory module) is detected in the background by hardware.
> > > +
> > > +1: Kill all processes that have the corrupted page mapped as soon as the
> > > +corruption is detected.
> > > +
> > > +0: Only unmap the page from all processes and only kill a process
> > > +who tries to access it.
> >
> > Note that
> > - no process will be killed if the page data is clean and can be
> > safely reloaded from disk
> > - pages in swap cache is always late killed.
>
> I clarified that

Thanks,
Fengguang