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SubjectRe: out_of_memory() heuristic broken for different mem configurations


On Tue, 6 Nov 2001, Marcelo Tosatti wrote:

>
> > With the more aggressive max_mapped, the oom failure count could be
> > dropped to something smaller, as a false positive from shrink_caches
> > should be fairly rare. I don't think it needs to be tunable on memory
> > size, I just didn't even try any other values on my machines (I noticed
> > that the old values were too high once max_mapped was upped and the swap
> > cache reclaiming was re-done, but I didn't try if five seconds and ten
> > failures was any better than 10 seconds and five failures, for example)
>
> Ok, I'll take a careful look at shrink_cache()/try_to_free_pages() path
> later and find out "saner" magic numbers for big/small memory workloads.

Ok, I found it. The problem is that swap_out() tries to scan the _whole_
address space looking for pte's to deactivate, and try_to_swap_out() does
not return a value indicating the lack of swap space, so at each
swap_out() call we simply loop around the whole VM when there is no swap
space available.

Here goes the tested fix.


--- linux.orig/mm/vmscan.c Sun Nov 4 22:54:44 2001
+++ linux/mm/vmscan.c Tue Nov 6 16:06:08 2001
@@ -36,7 +36,8 @@
/*
* The swap-out function returns 1 if it successfully
* scanned all the pages it was asked to (`count').
- * It returns zero if it couldn't do anything,
+ * It returns zero if it couldn't free the given pte or -1
+ * if there was no swap space left.
*
* rss may decrease because pages are shared, but this
* doesn't count as having freed a page.
@@ -142,7 +143,7 @@
/* No swap space left */
set_pte(page_table, pte);
UnlockPage(page);
- return 0;
+ return -1;
}

/* mm->page_table_lock is held. mmap_sem is not held */
@@ -170,7 +171,12 @@
struct page *page = pte_page(*pte);

if (VALID_PAGE(page) && !PageReserved(page)) {
- count -= try_to_swap_out(mm, vma, address, pte, page, classzone);
+ int ret = try_to_swap_out(mm, vma, address, pte, page, classzone);
+ if (ret < 0)
+ return ret;
+
+ count -= ret;
+
if (!count) {
address += PAGE_SIZE;
break;
@@ -205,7 +211,11 @@
end = pgd_end;

do {
- count = swap_out_pmd(mm, vma, pmd, address, end, count, classzone);
+ int ret = swap_out_pmd(mm, vma, pmd, address, end, count, classzone);
+
+ if (ret < 0)
+ return ret;
+ count = ret;
if (!count)
break;
address = (address + PMD_SIZE) & PMD_MASK;
@@ -230,7 +240,10 @@
if (address >= end)
BUG();
do {
- count = swap_out_pgd(mm, vma, pgdir, address, end, count, classzone);
+ int ret = swap_out_pgd(mm, vma, pgdir, address, end, count, classzone);
+ if (ret < 0)
+ return ret;
+ count = ret;
if (!count)
break;
address = (address + PGDIR_SIZE) & PGDIR_MASK;
@@ -287,7 +300,7 @@
static int FASTCALL(swap_out(unsigned int priority, unsigned int gfp_mask, zone_t * classzone));
static int swap_out(unsigned int priority, unsigned int gfp_mask, zone_t * classzone)
{
- int counter, nr_pages = SWAP_CLUSTER_MAX;
+ int counter, nr_pages = SWAP_CLUSTER_MAX, ret;
struct mm_struct *mm;

counter = mmlist_nr;
@@ -311,9 +324,15 @@
atomic_inc(&mm->mm_users);
spin_unlock(&mmlist_lock);

- nr_pages = swap_out_mm(mm, nr_pages, &counter, classzone);
+ ret = swap_out_mm(mm, nr_pages, &counter, classzone);

mmput(mm);
+
+ /* No more swap space ? */
+ if (ret < 0)
+ return nr_pages;
+
+ nr_pages = ret;

if (!nr_pages)
return 1;
-
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