[PATCH 1/6] MM: improve documentation for __GFP_NOFAIL
From: NeilBrown <hidden>
Date: 2021-09-14 00:27:55
Also in:
linux-fsdevel, linux-mm, linux-nfs, linux-xfs, lkml
Subsystem:
documentation, memory management - core, memory management - page allocator, the rest · Maintainers:
Jonathan Corbet, Andrew Morton, David Hildenbrand, Vlastimil Babka, Linus Torvalds
__GFP_NOFAIL is documented both in gfp.h and memory-allocation.rst. The details are not entirely consistent. This patch ensures both places state that: - there is a cost potentially imposed on other subsystems - it should only be used when there is no real alternative - it is preferable to an endless loop - it is strongly discourages for costly-order allocations. Signed-off-by: NeilBrown <redacted> --- Documentation/core-api/memory-allocation.rst | 9 ++++++++- include/linux/gfp.h | 4 ++++ 2 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
index 5954ddf6ee13..9458ce72d31c 100644
--- a/Documentation/core-api/memory-allocation.rst
+++ b/Documentation/core-api/memory-allocation.rst@@ -126,7 +126,14 @@ or another request. * ``GFP_KERNEL | __GFP_NOFAIL`` - overrides the default allocator behavior and all allocation requests will loop endlessly until they succeed. - This might be really dangerous especially for larger orders. + The allocator may provide access to memory that would otherwise be + reserved in order to satisfy this allocation which might adversely + affect other subsystems. So it should only be used when there is no + reasonable failure policy and when the memory is likely to be freed + again in the near future. Its use is strong discourage (via a + WARN_ON) for allocations larger than ``PAGE_ALLOC_COSTLY_ORDER``. + While this flag is best avoided, it is still preferable to endless + loops around the allocator. Selecting memory allocator ==========================
diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 55b2ec1f965a..101479373738 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h@@ -209,6 +209,10 @@ struct vm_area_struct; * used only when there is no reasonable failure policy) but it is * definitely preferable to use the flag rather than opencode endless * loop around allocator. + * Use of this flag may provide access to memory which would otherwise be + * reserved. As such it must be understood that there can be a cost imposed + * on other subsystems as well as the obvious cost of placing the calling + * thread in an uninterruptible indefinite wait. * Using this flag for costly allocations is _highly_ discouraged. */ #define __GFP_IO ((__force gfp_t)___GFP_IO)