On Tue 14-09-21 10:13:04, Neil Brown wrote: > __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. > Yes this is a useful addition to the documentation. Thanks! > Signed-off-by: NeilBrown <neilb@xxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> > --- > 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) > -- Michal Hocko SUSE Labs
