linux-kernel - [PATCH v3 3/3] mm/vmalloc: add more comments to the adjust_va_to_fit

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Date:   Wed, 16 Oct 2019 11:54:38 +0200
From:   "Uladzislau Rezki (Sony)" <urezki@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Daniel Wagner <dwagner@...e.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Hillf Danton <hdanton@...a.com>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [PATCH v3 3/3] 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.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 mm/vmalloc.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 593bf554518d..2290a0d270e4 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -969,6 +969,19 @@ adjust_va_to_fit_type(struct vmap_area *va,
 			 * 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)
-- 
2.20.1