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SubjectRe: out_of_memory called to often

Jason Li wrote:

>Hi,
>
>We are running an embedded linux (2.4.19) with no swap
>sapce. And we haven't applied any rmap yet.
>
I believe you may be seeing the same issue I brought up in a previous
email. Have you tried what I suggested? I've appended the email to this
message with out the test code.

Ross


I've verified this in 2.4.21-pre5 by code inspection and can trigger the
problem on 2.4.18. It appears to have been fixed in 2.5.

The folowing code vmscan.c assumes that there is available swap space.

/*
* this is the non-racy check for busy page.
*/
if (!page->mapping || !is_page_cache_freeable(page)) {
spin_unlock(&pagecache_lock);
UnlockPage(page);
page_mapped:
if (--max_mapped >= 0)
continue;

/*
* Alert! We've found too many mapped pages on the
* inactive list, so we start swapping out now!
*/
spin_unlock(&pagemap_lru_lock);
swap_out(priority, gfp_mask, classzone);
return nr_pages;
}


If there is no swap space, then unfreeable pages are left on the
inactive queue and the vmtree is walked rather than going through the
rest of the inactive queue. I believe something like
/*
* this is the non-racy check for busy page.
*/
if (!page->mapping || !is_page_cache_freeable(page)) {
spin_unlock(&pagecache_lock);
UnlockPage(page);
page_mapped:
/* If we don't have any swap space left, there
is no reason to worry about pages that do
not have swap associated with them, there
is nothing we can do about it. */
if (!page->mapping && !swap_avail()) {
/* Let's make the page active since we
cannot swap it out. It get's it off
the inactive list. */
spin_unlock(&pagemap_lru_lock);
activate_page(page);
ClearPageReferenced(page);
spin_lock(&pagemap_lru_lock);
continue;
}
if (--max_mapped >= 0)
continue;

/*
* Alert! We've found too many mapped pages on the
* inactive list, so we start swapping out now!
*/
spin_unlock(&pagemap_lru_lock);
swap_out(priority, gfp_mask, classzone);
return nr_pages;
}

will work better when there is no swap space available. If this change
is made, it may also be necessary to limit refill_inactive to prevent it
from using too much cpu. This bug can be triggered with the attached
code and the correct parameters. In particular on a 3 gigabyte machine
with no swap,

for i in $(seq 0 9); do dd if=/dev/zero of=file$i bs=1024k count=512; done
killmm 1032735283 2 9

Usually causes an out of memory error when there is hundreds of
megabytes of cache.


Ross

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