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Date:   Tue, 22 Nov 2022 12:30:22 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Seth Jennings <sjenning@redhat.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Vitaly Wool <vitaly.wool@konsulko.com>,
        Nhat Pham <nphamcs@gmail.com>,
        Minchan Kim <minchan@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] zswap: do not allocate from atomic pool
Message-ID: <Y3xCTr6ikbtcUr/y@google.com>
References: <20221122013338.3696079-1-senozhatsky@chromium.org>
 <Y3w/DFTAypX7L2mp@cmpxchg.org>
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On (22/11/21 22:16), Johannes Weiner wrote:
> On Tue, Nov 22, 2022 at 10:33:38AM +0900, Sergey Senozhatsky wrote:
> > zswap_frontswap_load() should be called from preemptible
> > context (we even call mutex_lock() there) and it does not
> > look like we need to do GFP_ATOMIC allocaion for temp
> > buffer there. Use GFP_KERNEL instead.
> > 
> > Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> > ---
> >  mm/zswap.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 2d69c1d678fe..f6c89049cf70 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1314,7 +1314,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> >  	}
> >  
> >  	if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
> > -		tmp = kmalloc(entry->length, GFP_ATOMIC);
> > +		tmp = kmalloc(entry->length, GFP_KERNEL);
> 
> There is another one in zswap_writeback_entry() that seems equally
> arbitrary. They came in through the same commit, with no further
> explanation as to this choice. Do you want to pick that up too?

Yup, you patch it in https://lore.kernel.org/lkml/20221119001536.2086599-2-nphamcs@gmail.com/
and I guess it's there just by accident. We probably want a separate
patch instead that touches those GFP_ATOMIC allocations in both places.

So I have it like this at present

---

From 66e4acffc0926498f818d11f2f57b9c772131f6e Mon Sep 17 00:00:00 2001
From: Sergey Senozhatsky <senozhatsky@chromium.org>
Date: Tue, 22 Nov 2022 10:26:13 +0900
Subject: [PATCH] zswap: do not allocate from atomic pool

zswap_frontswap_load() should be called from preemptible
context (we even call mutex_lock() there) and it does not
look like we need to do GFP_ATOMIC allocaion for temp
buffer. The same applies to zswap_writeback_entry().

Use GFP_KERNEL for temporary buffer allocation in both
cases.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 mm/zpool.c | 7 +++++++
 mm/zswap.c | 4 ++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/zpool.c b/mm/zpool.c
index 68facc193496..f46c0d5e766c 100644
--- a/mm/zpool.c
+++ b/mm/zpool.c
@@ -387,6 +387,13 @@ bool zpool_evictable(struct zpool *zpool)
  * zpool_can_sleep_mapped - Test if zpool can sleep when do mapped.
  * @zpool:	The zpool to test
  *
+ * Some allocators enter non-preemptible context in ->map() callback (e.g.
+ * disable pagefaults) and exit that context in ->unmap(), which limits what
+ * we can do with the mapped object. For instance, we cannot wait for
+ * asynchronous crypto API to decompress such an object or take mutexes
+ * since those will call into the scheduler. This function tells us whether
+ * we use such an allocator.
+ *
  * Returns: true if zpool can sleep; false otherwise.
  */
 bool zpool_can_sleep_mapped(struct zpool *zpool)
diff --git a/mm/zswap.c b/mm/zswap.c
index 2d48fd59cc7a..3019f0bde194 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -958,7 +958,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 	};
 
 	if (!zpool_can_sleep_mapped(pool)) {
-		tmp = kmalloc(PAGE_SIZE, GFP_ATOMIC);
+		tmp = kmalloc(PAGE_SIZE, GFP_KERNEL);
 		if (!tmp)
 			return -ENOMEM;
 	}
@@ -1311,7 +1311,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 	}
 
 	if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
-		tmp = kmalloc(entry->length, GFP_ATOMIC);
+		tmp = kmalloc(entry->length, GFP_KERNEL);
 		if (!tmp) {
 			ret = -ENOMEM;
 			goto freeentry;
-- 
2.38.1.584.g0f3c55d4c2-goog