[PATCHv2 6/9] zram: permit reclaim in zstd custom allocator

[Sergey Senozhatsky <senozhatsky@chromium.org>]

When configured with pre-trained compression/decompression
dictionary support, zstd requires custom memory allocator,
which it calls internally from compression()/decompression()
routines.  This was a tad problematic, because that would
mean allocation from atomic context (either under entry
spin-lock, or per-CPU local-lock or both).  Now, with
non-atomic zram write(), those limitations are relaxed and
we can allow direct and indirect reclaim during allocations.

The tricky part is zram read() path, which is still atomic in
one particular case (read_compressed_page()), due to zsmalloc
handling of object mapping.  However, in zram in order to read()
something one has to write() it first, and write() is when zstd
allocates required internal state memory, and write() path is
non-atomic.  Because of this write() allocation, in theory, zstd
should not call its allocator from the atomic read() path.  Keep
the non-preemptible branch, just in case if zstd allocates memory
from read(), but WARN_ON_ONCE() if it happens.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 drivers/block/zram/backend_zstd.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/drivers/block/zram/backend_zstd.c b/drivers/block/zram/backend_zstd.c
index 1184c0036f44..53431251ea62 100644
--- a/drivers/block/zram/backend_zstd.c
+++ b/drivers/block/zram/backend_zstd.c
@@ -24,19 +24,14 @@ struct zstd_params {
 /*
  * For C/D dictionaries we need to provide zstd with zstd_custom_mem,
  * which zstd uses internally to allocate/free memory when needed.
- *
- * This means that allocator.customAlloc() can be called from zcomp_compress()
- * under local-lock (per-CPU compression stream), in which case we must use
- * GFP_ATOMIC.
- *
- * Another complication here is that we can be configured as a swap device.
  */
 static void *zstd_custom_alloc(void *opaque, size_t size)
 {
-	if (!preemptible())
+	/* Technically this should not happen */
+	if (WARN_ON_ONCE(!preemptible()))
 		return kvzalloc(size, GFP_ATOMIC);
 
-	return kvzalloc(size, __GFP_KSWAPD_RECLAIM | __GFP_NOWARN);
+	return kvzalloc(size, GFP_NOIO | __GFP_NOWARN);
 }
 
 static void zstd_custom_free(void *opaque, void *address)
-- 
2.48.1.262.g85cc9f2d1e-goog