drm/ttm: Avoid memory allocation from shrinker functions.

Andrew Morton wrote:
> On Wed, 12 Nov 2014 13:08:55 +0900 Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> > Andrew Morton wrote:
> > > Poor ttm guys - this is a bit of a trap we set for them.
> >
> > Commit a91576d7916f6cce ("drm/ttm: Pass GFP flags in order to avoid deadlock.")
> > changed to use sc->gfp_mask rather than GFP_KERNEL.
> >
> > -       pages_to_free = kmalloc(npages_to_free * sizeof(struct page *),
> > -                       GFP_KERNEL);
> > +       pages_to_free = kmalloc(npages_to_free * sizeof(struct page *), gfp);
> >
> > But this bug is caused by sc->gfp_mask containing some flags which are not
> > in GFP_KERNEL, right? Then, I think
> >
> > -       pages_to_free = kmalloc(npages_to_free * sizeof(struct page *), gfp);
> > +       pages_to_free = kmalloc(npages_to_free * sizeof(struct page *), gfp & GFP_KERNEL);
> >
> > would hide this bug.
> >
> > But I think we should use GFP_ATOMIC (or drop __GFP_WAIT flag)
>
> Well no - ttm_page_pool_free() should stop calling kmalloc altogether.
> Just do
>
> 	struct page *pages_to_free[16];
>
> and rework the code to free 16 pages at a time.  Easy.

Well, ttm code wants to process 512 pages at a time for performance.
Memory footprint increased by 512 * sizeof(struct page *) buffer is
only 4096 bytes. What about using static buffer like below?
----------
>From d3cb5393c9c8099d6b37e769f78c31af1541fe8c Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Thu, 13 Nov 2014 22:21:54 +0900
Subject: [PATCH] drm/ttm: Avoid memory allocation from shrinker functions.

Commit a91576d7916f6cce ("drm/ttm: Pass GFP flags in order to avoid
deadlock.") caused BUG_ON() due to sc->gfp_mask containing flags
which are not in GFP_KERNEL.

  https://bugzilla.kernel.org/show_bug.cgi?id=87891

Changing from sc->gfp_mask to (sc->gfp_mask & GFP_KERNEL) would
avoid the BUG_ON(), but avoiding memory allocation from shrinker
function is better and reliable fix.

Shrinker function is already serialized by global lock, and
clean up function is called after shrinker function is unregistered.
Thus, we can use static buffer when called from shrinker function
and clean up function.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: stable <stable@kernel.org> [2.6.35+]
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 15 insertions, 11 deletions

diff --git a/drivers/gpu/drm/ttm/ttm_page_alloc.c b/drivers/gpu/drm/ttm/ttm_page_alloc.c
index 09874d695188..025c429050c0 100644
--- a/drivers/gpu/drm/ttm/ttm_page_alloc.c
+++ b/drivers/gpu/drm/ttm/ttm_page_alloc.c
@@ -297,11 +297,12 @@ static void ttm_pool_update_free_locked(struct ttm_page_pool *pool,
  *
  * @pool: to free the pages from
  * @free_all: If set to true will free all pages in pool
- * @gfp: GFP flags.
+ * @use_static: Safe to use static buffer
  **/
 static int ttm_page_pool_free(struct ttm_page_pool *pool, unsigned nr_free,
-			      gfp_t gfp)
+			      bool use_static)
 {
+	static struct page *static_buf[NUM_PAGES_TO_ALLOC];
 	unsigned long irq_flags;
 	struct page *p;
 	struct page **pages_to_free;
@@ -311,7 +312,11 @@ static int ttm_page_pool_free(struct ttm_page_pool *pool, unsigned nr_free,
 	if (NUM_PAGES_TO_ALLOC < nr_free)
 		npages_to_free = NUM_PAGES_TO_ALLOC;
 
-	pages_to_free = kmalloc(npages_to_free * sizeof(struct page *), gfp);
+	if (use_static)
+		pages_to_free = static_buf;
+	else
+		pages_to_free = kmalloc(npages_to_free * sizeof(struct page *),
+					GFP_KERNEL);
 	if (!pages_to_free) {
 		pr_err("Failed to allocate memory for pool free operation\n");
 		return 0;
@@ -374,7 +379,8 @@ restart:
 	if (freed_pages)
 		ttm_pages_put(pages_to_free, freed_pages);
 out:
-	kfree(pages_to_free);
+	if (pages_to_free != static_buf)
+		kfree(pages_to_free);
 	return nr_free;
 }
 
@@ -383,8 +389,6 @@ out:
  *
  * XXX: (dchinner) Deadlock warning!
  *
- * We need to pass sc->gfp_mask to ttm_page_pool_free().
- *
  * This code is crying out for a shrinker per pool....
  */
 static unsigned long
@@ -407,8 +411,8 @@ ttm_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 		if (shrink_pages == 0)
 			break;
 		pool = &_manager->pools[(i + pool_offset)%NUM_POOLS];
-		shrink_pages = ttm_page_pool_free(pool, nr_free,
-						  sc->gfp_mask);
+		/* OK to use static buffer since global mutex is held. */
+		shrink_pages = ttm_page_pool_free(pool, nr_free, true);
 		freed += nr_free - shrink_pages;
 	}
 	mutex_unlock(&lock);
@@ -710,7 +714,7 @@ static void ttm_put_pages(struct page **pages, unsigned npages, int flags,
 	}
 	spin_unlock_irqrestore(&pool->lock, irq_flags);
 	if (npages)
-		ttm_page_pool_free(pool, npages, GFP_KERNEL);
+		ttm_page_pool_free(pool, npages, false);
 }
 
 /*
@@ -849,9 +853,9 @@ void ttm_page_alloc_fini(void)
 	pr_info("Finalizing pool allocator\n");
 	ttm_pool_mm_shrink_fini(_manager);
 
+	/* OK to use static buffer since global mutex is no longer used. */
 	for (i = 0; i < NUM_POOLS; ++i)
-		ttm_page_pool_free(&_manager->pools[i], FREE_ALL_PAGES,
-				   GFP_KERNEL);
+		ttm_page_pool_free(&_manager->pools[i], FREE_ALL_PAGES, true);
 
 	kobject_put(&_manager->kobj);
 	_manager = NULL;