linux-kernel - [PATCH v2] mm: remove unnecessary __get_user_pages

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Message-Id: <20161026092548.12712-1-lstoakes@gmail.com>
Date:   Wed, 26 Oct 2016 10:25:48 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     linux-mm@...ck.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, Jan Kara <jack@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lorenzo Stoakes <lstoakes@...il.com>
Subject: [PATCH v2] mm: remove unnecessary __get_user_pages_unlocked() calls

In hva_to_pfn_slow() we are able to replace __get_user_pages_unlocked() with
get_user_pages_unlocked() since we can now pass gup_flags.

In async_pf_execute() we need to pass different tsk, mm arguments so
get_user_pages_remote() is the sane replacement here (having added manual
acquisition and release of mmap_sem.)

Since we pass a NULL pages parameter the subsequent call to
__get_user_pages_locked() will have previously bailed any attempt at
VM_FAULT_RETRY, so we do not change this behaviour by using
get_user_pages_remote() which does not invoke VM_FAULT_RETRY logic at all.

Additionally get_user_pages_remote() reintroduces use of the FOLL_TOUCH
flag. However, this flag was originally silently dropped by 1e9877902dc7e
("mm/gup: Introduce get_user_pages_remote()"), so this appears to have been
unintentional and reintroducing it is therefore not an issue.

Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
---
v2: update description to reference dropped FOLL_TOUCH flag

 virt/kvm/async_pf.c | 7 ++++---
 virt/kvm/kvm_main.c | 5 ++---
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
index 8035cc1..e8c832c 100644
--- a/virt/kvm/async_pf.c
+++ b/virt/kvm/async_pf.c
@@ -82,10 +82,11 @@ static void async_pf_execute(struct work_struct *work)
 	/*
 	 * This work is run asynchromously to the task which owns
 	 * mm and might be done in another context, so we must
-	 * use FOLL_REMOTE.
+	 * access remotely.
 	 */
-	__get_user_pages_unlocked(NULL, mm, addr, 1, NULL,
-			FOLL_WRITE | FOLL_REMOTE);
+	down_read(&mm->mmap_sem);
+	get_user_pages_remote(NULL, mm, addr, 1, FOLL_WRITE, NULL, NULL);
+	up_read(&mm->mmap_sem);

 	kvm_async_page_present_sync(vcpu, apf);

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2907b7b..c45d951 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1415,13 +1415,12 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
 		npages = get_user_page_nowait(addr, write_fault, page);
 		up_read(&current->mm->mmap_sem);
 	} else {
-		unsigned int flags = FOLL_TOUCH | FOLL_HWPOISON;
+		unsigned int flags = FOLL_HWPOISON;

 		if (write_fault)
 			flags |= FOLL_WRITE;

-		npages = __get_user_pages_unlocked(current, current->mm, addr, 1,
-						   page, flags);
+		npages = get_user_pages_unlocked(addr, 1, page, flags);
 	}
 	if (npages != 1)
 		return npages;
--
2.10.1