From: "Uladzislau Rezki (Sony)" <urezki@xxxxxxxxx> Subject: mm/vmalloc: add more comments to the adjust_va_to_fit_type() When fit type is NE_FIT_TYPE there is a need in one extra object. Usually the "ne_fit_preload_node" per-CPU variable has it and there is no need in GFP_NOWAIT allocation, but there are exceptions. This commit just adds more explanations, as a result giving answers on questions like when it can occur, how often, under which conditions and what happens if GFP_NOWAIT gets failed. Link: http://lkml.kernel.org/r/20191016095438.12391-3-urezki@xxxxxxxxx Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Cc: Daniel Wagner <dwagner@xxxxxxx> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Uladzislau Rezki <urezki@xxxxxxxxx> Cc: Hillf Danton <hdanton@xxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Oleksiy Avramchenko <oleksiy.avramchenko@xxxxxxxxxxxxxx> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/vmalloc.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) --- a/mm/vmalloc.c~mm-vmalloc-add-more-comments-to-the-adjust_va_to_fit_type +++ a/mm/vmalloc.c @@ -968,6 +968,19 @@ adjust_va_to_fit_type(struct vmap_area * * There are a few exceptions though, as an example it is * a first allocation (early boot up) when we have "one" * big free space that has to be split. + * + * Also we can hit this path in case of regular "vmap" + * allocations, if "this" current CPU was not preloaded. + * See the comment in alloc_vmap_area() why. If so, then + * GFP_NOWAIT is used instead to get an extra object for + * split purpose. That is rare and most time does not + * occur. + * + * What happens if an allocation gets failed. Basically, + * an "overflow" path is triggered to purge lazily freed + * areas to free some memory, then, the "retry" path is + * triggered to repeat one more time. See more details + * in alloc_vmap_area() function. */ lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT); if (!lva) _
