Re: [PATCH V4 3/6] mm: gup: Add mm_lock_present()

Kirill A. Shutemov
2015-07-22T11:13:24Z

On Tue, Jul 21, 2015 at 03:59:38PM -0400, Eric B Munson wrote:
> The upcoming mlock(MLOCK_ONFAULT) implementation will need a way to
> request that all present pages in a range are locked without faulting in
> pages that are not present. This logic is very close to what the
> __mm_populate() call handles without faulting pages so the patch pulls
> out the pieces that can be shared and adds mm_lock_present() to gup.c.
> The following patch will call it from do_mlock() when MLOCK_ONFAULT is
> specified.
>
> Signed-off-by: Eric B Munson <emunson@akamai.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
> mm/gup.c | 172 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 157 insertions(+), 15 deletions(-)

I don't like that you've copy-pasted a lot of code. I think it can be
solved with new foll flags.

Totally untested patch below split out mlock part of FOLL_POPULATE into
new FOLL_MLOCK flag. FOLL_POPULATE | FOLL_MLOCK will do what currently
FOLL_POPULATE does. The new MLOCK_ONFAULT can use just FOLL_MLOCK. It will
not trigger fault in.

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c3a2b37365f6..c3834cddfcc7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2002,6 +2002,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
#define FOLL_NUMA 0x200 /* force NUMA hinting page fault */
#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
+#define FOLL_MLOCK 0x1000 /* mlock the page if the VMA is VM_LOCKED */

typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
void *data);
diff --git a/mm/gup.c b/mm/gup.c
index a798293fc648..4c7ff23947b9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -129,7 +129,7 @@ retry:
*/
mark_page_accessed(page);
}
- if ((flags & FOLL_POPULATE) && (vma->vm_flags & VM_LOCKED)) {
+ if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
/*
* The preliminary mapping check is mainly to avoid the
* pointless overhead of lock_page on the ZERO_PAGE
@@ -299,6 +299,9 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
unsigned int fault_flags = 0;
int ret;

+ /* mlock present pages, but not fault in new one */
+ if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
+ return -ENOENT;
/* For mm_populate(), just skip the stack guard page. */
if ((*flags & FOLL_POPULATE) &&
(stack_guard_page_start(vma, address) ||
@@ -890,7 +893,7 @@ long populate_vma_page_range(struct vm_area_struct *vma,
VM_BUG_ON_VMA(end > vma->vm_end, vma);
VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);

- gup_flags = FOLL_TOUCH | FOLL_POPULATE;
+ gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK;
/*
* We want to touch writable mappings with a write fault in order
* to break COW, except for shared mappings because these don't COW
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8f9a334a6c66..9eeb3bd304fc 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1306,7 +1306,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
pmd, _pmd, 1))
update_mmu_cache_pmd(vma, addr, pmd);
}
- if ((flags & FOLL_POPULATE) && (vma->vm_flags & VM_LOCKED)) {
+ if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
if (page->mapping && trylock_page(page)) {
lru_add_drain();
if (page->mapping)

--
Kirill A. Shutemov