tree 408abec084f6efe3c1c9af1683236e3951b98b6c
parent 2ca84728aed3909fae3672ee2c069b17aca78afd
author Laurent Dufour <ldufour@linux.vnet.ibm.com> 1523975603 +0200
committer Vinayak Menon <vinmenon@codeaurora.org> 1530193444 +0530

mm: protect mm_rb tree with a rwlock

This change is inspired by the Peter's proposal patch [1] which was
protecting the VMA using SRCU. Unfortunately, SRCU is not scaling well in
that particular case, and it is introducing major performance degradation
due to excessive scheduling operations.

To allow access to the mm_rb tree without grabbing the mmap_sem, this patch
is protecting it access using a rwlock.  As the mm_rb tree is a O(log n)
search it is safe to protect it using such a lock.  The VMA cache is not
protected by the new rwlock and it should not be used without holding the
mmap_sem.

To allow the picked VMA structure to be used once the rwlock is released, a
use count is added to the VMA structure. When the VMA is allocated it is
set to 1.  Each time the VMA is picked with the rwlock held its use count
is incremented. Each time the VMA is released it is decremented. When the
use count hits zero, this means that the VMA is no more used and should be
freed.

This patch is preparing for 2 kind of VMA access :
 - as usual, under the control of the mmap_sem,
 - without holding the mmap_sem for the speculative page fault handler.

Access done under the control the mmap_sem doesn't require to grab the
rwlock to protect read access to the mm_rb tree, but access in write must
be done under the protection of the rwlock too. This affects inserting and
removing of elements in the RB tree.

The patch is introducing 2 new functions:
 - vma_get() to find a VMA based on an address by holding the new rwlock.
 - vma_put() to release the VMA when its no more used.
These services are designed to be used when access are made to the RB tree
without holding the mmap_sem.

When a VMA is removed from the RB tree, its vma->vm_rb field is cleared and
we rely on the WMB done when releasing the rwlock to serialize the write
with the RMB done in a later patch to check for the VMA's validity.

When free_vma is called, the file associated with the VMA is closed
immediately, but the policy and the file structure remained in used until
the VMA's use count reach 0, which may happens later when exiting an
in progress speculative page fault.

[1] https://patchwork.kernel.org/patch/5108281/

Change-Id: I9ecc922b8efa4b28975cc6a8e9531284c24ac14e
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Patch-mainline: linux-mm @ Tue, 17 Apr 2018 16:33:23
[vinmenon@codeaurora.org: fix the return of put_vma]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>