[RFC][PATCH] shrink_mmap avoid list_del (Was: Re: [PATCH] Recent VM fiasco - fixed)

Roger Larsson
2000-05-12T02:51:42Z

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Hi,

I tried to find a way to walk the lru list without list_del.

Here is my patch:
- not compiled nor run (low on HD...)

Could something like this be used?
If no, why not?

/RogerL

Linus Torvalds wrote:
>
> On Thu, 11 May 2000, Simon Kirby wrote:
> >
> > Hrm! pre7 release seems to be even better. 113 vmstat-line-seconds now
> > (yes, I know this isn't a very scientific testing method :)). Second try
> > was 114 vmstat-line-seconds. classzone-27 did it in 107, so that's not
> > very far off! Also, it swapped much less this time, and used less CPU.
> > vmstat output attached.
>
> The final pre7 did something that I'm not entirely excited about, but that
> kind of makes sense at least from a CPU standpoint (as the SGI people have
> repeated multiple times). What the real pre7 does is to just move any page
> that has problems getting free'd to the head of the LRU list, so that we
> won't try it immediately the next time. This way we don't test the same
> pages over and over again when they are either shared, in the wrong zone,
> or have dirty/locked buffers.
>
> It means that the "LRU" is less LRU, but you could see it as a "how hard
> do we want to free this" pressure-based system that really a least
> recently _used_ system. And it avoids the "repeat the whole thing on the
> same page" issue. And it looks like it behaves reasonably well, while
> saving a lot of CPU.
>
> Knock wood.
>
> I'm still considering the pre7 as more a "ok, I tried to get rid of the
> cruft" thing. Most of the special case code that has accumulated lately is
> gone. We can start adding stuff back now, I'm happy that the basics are
> reasonably clean.
>
> I think Ingo already posted a very valid concern about high-memory
> machines, and there are other issues we should look at. I just want to be
> in a position where we can look at the code and say "we do X because Y",
> rather than a collection of random tweaks that just happens to work.
>
> Linus
>
> --
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--
Home page:
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diff -Naur linux-2.3-pre9--/mm/filemap.c linux-2.3/mm/filemap.c
--- linux-2.3-pre9--/mm/filemap.c Fri May 12 02:42:19 2000
+++ linux-2.3/mm/filemap.c Fri May 12 04:28:30 2000
@@ -236,7 +236,6 @@
int shrink_mmap(int priority, int gfp_mask)
{
int ret = 0, count;
- LIST_HEAD(old);
struct list_head * page_lru, * dispose;
struct page * page = NULL;

@@ -244,26 +243,29 @@

/* we need pagemap_lru_lock for list_del() ... subtle code below */
spin_lock(&pagemap_lru_lock);
- while (count > 0 && (page_lru = lru_cache.prev) != &lru_cache) {
+ page_lru = &lru_cache;
+ while (count > 0 && (page_lru = page_lru->prev) != &lru_cache) {
page = list_entry(page_lru, struct page, lru);
- list_del(page_lru);

dispose = &lru_cache;
if (PageTestandClearReferenced(page))
goto dispose_continue;

count--;
- dispose = &old;
+
+ dispose = NULL;

/*
* Avoid unscalable SMP locking for pages we can
* immediate tell are untouchable..
*/
if (!page->buffers && page_count(page) > 1)
- goto dispose_continue;
+ continue;

+ /* Lock this lru page, reentrant
+ * will be disposed correctly when unlocked */
if (TryLockPage(page))
- goto dispose_continue;
+ continue;

/* Release the pagemap_lru lock even if the page is not yet
queued in any lru queue since we have just locked down
@@ -281,7 +283,7 @@
*/
if (page->buffers) {
if (!try_to_free_buffers(page))
- goto unlock_continue;
+ goto page_unlock_continue;
/* page was locked, inode can't go away under us */
if (!page->mapping) {
atomic_dec(&buffermem_pages);
@@ -336,27 +338,43 @@

cache_unlock_continue:
spin_unlock(&pagecache_lock);
-unlock_continue:
+page_unlock_continue:
spin_lock(&pagemap_lru_lock);
UnlockPage(page);
put_page(page);
+ continue;
+
dispose_continue:
- list_add(page_lru, dispose);
- }
- goto out;
+ /* have the pagemap_lru_lock, lru cannot change */
+ {
+ struct list_head * page_lru_to_move = page_lru;
+ page_lru = page_lru->next; /* continues with page_lru.prev */
+ list_del(page_lru_to_move);
+ list_add(page_lru_to_move, dispose);
+ }
+ continue;

made_inode_progress:
- page_cache_release(page);
+ page_cache_release(page);
made_buffer_progress:
- UnlockPage(page);
- put_page(page);
- ret = 1;
- spin_lock(&pagemap_lru_lock);
- /* nr_lru_pages needs the spinlock */
- nr_lru_pages--;
+ /* like to have the lru lock before UnlockPage */
+ spin_lock(&pagemap_lru_lock);

-out:
- list_splice(&old, lru_cache.prev);
+ UnlockPage(page);
+ put_page(page);
+ ret++;
+
+ /* lru manipulation needs the spin lock */
+ {
+ struct list_head * page_lru_to_free = page_lru;
+ page_lru = page_lru->next; /* continues with page_lru.prev */
+ list_del(page_lru_to_free);
+ }
+
+ /* nr_lru_pages needs the spinlock */
+ nr_lru_pages--;
+
+ }

spin_unlock(&pagemap_lru_lock);

diff -Naur linux-2.3-pre9--/mm/vmscan.c linux-2.3/mm/vmscan.c
--- linux-2.3-pre9--/mm/vmscan.c Fri May 12 02:42:19 2000
+++ linux-2.3/mm/vmscan.c Fri May 12 04:32:16 2000
@@ -443,10 +443,9 @@

priority = 6;
do {
- while (shrink_mmap(priority, gfp_mask)) {
- if (!--count)
- goto done;
- }
+ count -= shrink_mmap(priority, gfp_mask);
+ if (count <= 0)
+ goto done;

/* Try to get rid of some shared memory pages.. */
if (gfp_mask & __GFP_IO) {
@@ -481,10 +480,9 @@
} while (--priority >= 0);

/* Always end on a shrink_mmap.. */
- while (shrink_mmap(0, gfp_mask)) {
- if (!--count)
- goto done;
- }
+ count -= shrink_mmap(priority, gfp_mask);
+ if (count <= 0)
+ goto done;

return 0;