Michal Hocko wrote: > On Mon 29-01-18 22:00:11, Zi Yan wrote: >> From: Zi Yan <zi.yan@xxxxxxxxxxxxxx> >> >> migrate_pages() requires at least down_read(mmap_sem) to protect >> related page tables and VMAs from changing. Let's do it in >> do_page_moves() for both do_move_pages_to_node() and >> add_page_for_migration(). >> >> Also add this lock requirement in the comment of migrate_pages(). > > This doesn't make much sense to me, to be honest. We are holding > mmap_sem for _read_ so we allow parallel updates like page faults > or unmaps. Therefore we are isolating pages prior to the migration. > > The sole purpose of the mmap_sem in add_page_for_migration is to protect > from vma going away _while_ need it to get the proper page. Then, I am wondering why we are holding mmap_sem when calling migrate_pages() in existing code. http://elixir.free-electrons.com/linux/latest/source/mm/migrate.c#L1576 > > Moving the lock up is just wrong because it allows caller to hold the > lock for way too long if a lot of pages is migrated. Not only that, > it is even incorrect because we are doing get_user() (aka page fault) > and while read lock recursion is OK, we might block and deadlock when > there is a writer pending. I haven't checked the current implementation > of semaphores but I believe we do not allow recursive locking. > Sorry, I missed that. If mmap_sem is not needed for migrate_pages(), please ignore this patch. >> Signed-off-by: Zi Yan <zi.yan@xxxxxxxxxxxxxx> >> --- >> mm/migrate.c | 13 +++++++++++-- >> 1 file changed, 11 insertions(+), 2 deletions(-) >> >> diff --git a/mm/migrate.c b/mm/migrate.c >> index 5d0dc7b85f90..52d029953c32 100644 >> --- a/mm/migrate.c >> +++ b/mm/migrate.c >> @@ -1354,6 +1354,9 @@ static int unmap_and_move_huge_page(new_page_t get_new_page, >> * or free list only if ret != 0. >> * >> * Returns the number of pages that were not migrated, or an error code. >> + * >> + * The caller must hold at least down_read(mmap_sem) for to-be-migrated pages >> + * to protect related page tables and VMAs from changing. >> */ >> int migrate_pages(struct list_head *from, new_page_t get_new_page, >> free_page_t put_new_page, unsigned long private, >> @@ -1457,6 +1460,12 @@ static int store_status(int __user *status, int start, int value, int nr) >> return 0; >> } >> >> +/* >> + * Migrates the pages from pagelist and put back those not migrated. >> + * >> + * The caller must at least hold down_read(mmap_sem), which is required >> + * for migrate_pages() >> + */ >> static int do_move_pages_to_node(struct mm_struct *mm, >> struct list_head *pagelist, int node) >> { >> @@ -1487,7 +1496,6 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr, >> unsigned int follflags; >> int err; >> >> - down_read(&mm->mmap_sem); >> err = -EFAULT; >> vma = find_vma(mm, addr); >> if (!vma || addr < vma->vm_start || !vma_migratable(vma)) >> @@ -1540,7 +1548,6 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr, >> */ >> put_page(page); >> out: >> - up_read(&mm->mmap_sem); >> return err; >> } >> >> @@ -1561,6 +1568,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes, >> >> migrate_prep(); >> >> + down_read(&mm->mmap_sem); >> for (i = start = 0; i < nr_pages; i++) { >> const void __user *p; >> unsigned long addr; >> @@ -1628,6 +1636,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes, >> if (!err) >> err = err1; >> out: >> + up_read(&mm->mmap_sem); >> return err; >> } >> >> -- >> 2.15.1 > -- Best Regards, Yan Zi
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