Re: [RFCv2 4/6] mm: factor out madvise's core functionality

On Fri, May 31, 2019 at 09:04:20AM +0200, Oleksandr Natalenko wrote:

On Fri, May 31, 2019 at 03:43:11PM +0900, Minchan Kim wrote:

quoted

This patch factor out madvise's core functionality so that upcoming
patch can reuse it without duplication. It shouldn't change any behavior.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 mm/madvise.c | 188 +++++++++++++++++++++++++++------------------------
 1 file changed, 101 insertions(+), 87 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 9d749a1420b4..466623ea8c36 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c

@@ -425,9 +425,10 @@ static int madvise_pageout_pte_range(pmd_t *pmd, unsigned long addr,
 	struct page *page;
 	int isolated = 0;
 	struct vm_area_struct *vma = walk->vma;
+	struct task_struct *task = walk->private;
 	unsigned long next;
 
-	if (fatal_signal_pending(current))
+	if (fatal_signal_pending(task))
 		return -EINTR;
 
 	next = pmd_addr_end(addr, end);

@@ -505,12 +506,14 @@ static int madvise_pageout_pte_range(pmd_t *pmd, unsigned long addr,
 }
 
 static void madvise_pageout_page_range(struct mmu_gather *tlb,
-			     struct vm_area_struct *vma,
-			     unsigned long addr, unsigned long end)
+				struct task_struct *task,
+				struct vm_area_struct *vma,
+				unsigned long addr, unsigned long end)
 {
 	struct mm_walk warm_walk = {
 		.pmd_entry = madvise_pageout_pte_range,
 		.mm = vma->vm_mm,
+		.private = task,
 	};
 
 	tlb_start_vma(tlb, vma);

@@ -519,9 +522,9 @@ static void madvise_pageout_page_range(struct mmu_gather *tlb,
 }
 
 
-static long madvise_pageout(struct vm_area_struct *vma,
-			struct vm_area_struct **prev,
-			unsigned long start_addr, unsigned long end_addr)
+static long madvise_pageout(struct task_struct *task,
+		struct vm_area_struct *vma, struct vm_area_struct **prev,
+		unsigned long start_addr, unsigned long end_addr)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	struct mmu_gather tlb;

@@ -532,7 +535,7 @@ static long madvise_pageout(struct vm_area_struct *vma,
 
 	lru_add_drain();
 	tlb_gather_mmu(&tlb, mm, start_addr, end_addr);
-	madvise_pageout_page_range(&tlb, vma, start_addr, end_addr);
+	madvise_pageout_page_range(&tlb, task, vma, start_addr, end_addr);
 	tlb_finish_mmu(&tlb, start_addr, end_addr);
 
 	return 0;

@@ -744,7 +747,8 @@ static long madvise_dontneed_single_vma(struct vm_area_struct *vma,
 	return 0;
 }
 
-static long madvise_dontneed_free(struct vm_area_struct *vma,
+static long madvise_dontneed_free(struct mm_struct *mm,
+				  struct vm_area_struct *vma,
 				  struct vm_area_struct **prev,
 				  unsigned long start, unsigned long end,
 				  int behavior)

@@ -756,8 +760,8 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
 	if (!userfaultfd_remove(vma, start, end)) {
 		*prev = NULL; /* mmap_sem has been dropped, prev is stale */
 
-		down_read(&current->mm->mmap_sem);
-		vma = find_vma(current->mm, start);
+		down_read(&mm->mmap_sem);
+		vma = find_vma(mm, start);
 		if (!vma)
 			return -ENOMEM;
 		if (start < vma->vm_start) {

@@ -804,7 +808,8 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
  * Application wants to free up the pages and associated backing store.
  * This is effectively punching a hole into the middle of a file.
  */
-static long madvise_remove(struct vm_area_struct *vma,
+static long madvise_remove(struct mm_struct *mm,
+				struct vm_area_struct *vma,
 				struct vm_area_struct **prev,
 				unsigned long start, unsigned long end)
 {

@@ -838,13 +843,13 @@ static long madvise_remove(struct vm_area_struct *vma,
 	get_file(f);
 	if (userfaultfd_remove(vma, start, end)) {
 		/* mmap_sem was not released by userfaultfd_remove() */
-		up_read(&current->mm->mmap_sem);
+		up_read(&mm->mmap_sem);
 	}
 	error = vfs_fallocate(f,
 				FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
 				offset, end - start);
 	fput(f);
-	down_read(&current->mm->mmap_sem);
+	down_read(&mm->mmap_sem);
 	return error;
 }
 

@@ -918,21 +923,23 @@ static int madvise_inject_error(int behavior,
 #endif
 
 static long
-madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev,
+madvise_vma(struct task_struct *task, struct mm_struct *mm,
+		struct vm_area_struct *vma, struct vm_area_struct **prev,
 		unsigned long start, unsigned long end, int behavior)
 {
 	switch (behavior) {
 	case MADV_REMOVE:
-		return madvise_remove(vma, prev, start, end);
+		return madvise_remove(mm, vma, prev, start, end);
 	case MADV_WILLNEED:
 		return madvise_willneed(vma, prev, start, end);
 	case MADV_COLD:
 		return madvise_cold(vma, prev, start, end);
 	case MADV_PAGEOUT:
-		return madvise_pageout(vma, prev, start, end);
+		return madvise_pageout(task, vma, prev, start, end);
 	case MADV_FREE:
 	case MADV_DONTNEED:
-		return madvise_dontneed_free(vma, prev, start, end, behavior);
+		return madvise_dontneed_free(mm, vma, prev, start,
+						end, behavior);
 	default:
 		return madvise_behavior(vma, prev, start, end, behavior);
 	}

@@ -976,68 +983,8 @@ madvise_behavior_valid(int behavior)
 	}
 }
 
-/*
- * The madvise(2) system call.
- *
- * Applications can use madvise() to advise the kernel how it should
- * handle paging I/O in this VM area.  The idea is to help the kernel
- * use appropriate read-ahead and caching techniques.  The information
- * provided is advisory only, and can be safely disregarded by the
- * kernel without affecting the correct operation of the application.
- *
- * behavior values:
- *  MADV_NORMAL - the default behavior is to read clusters.  This
- *		results in some read-ahead and read-behind.
- *  MADV_RANDOM - the system should read the minimum amount of data
- *		on any access, since it is unlikely that the appli-
- *		cation will need more than what it asks for.
- *  MADV_SEQUENTIAL - pages in the given range will probably be accessed
- *		once, so they can be aggressively read ahead, and
- *		can be freed soon after they are accessed.
- *  MADV_WILLNEED - the application is notifying the system to read
- *		some pages ahead.
- *  MADV_DONTNEED - the application is finished with the given range,
- *		so the kernel can free resources associated with it.
- *  MADV_FREE - the application marks pages in the given range as lazy free,
- *		where actual purges are postponed until memory pressure happens.
- *  MADV_REMOVE - the application wants to free up the given range of
- *		pages and associated backing store.
- *  MADV_DONTFORK - omit this area from child's address space when forking:
- *		typically, to avoid COWing pages pinned by get_user_pages().
- *  MADV_DOFORK - cancel MADV_DONTFORK: no longer omit this area when forking.
- *  MADV_WIPEONFORK - present the child process with zero-filled memory in this
- *              range after a fork.
- *  MADV_KEEPONFORK - undo the effect of MADV_WIPEONFORK
- *  MADV_HWPOISON - trigger memory error handler as if the given memory range
- *		were corrupted by unrecoverable hardware memory failure.
- *  MADV_SOFT_OFFLINE - try to soft-offline the given range of memory.
- *  MADV_MERGEABLE - the application recommends that KSM try to merge pages in
- *		this area with pages of identical content from other such areas.
- *  MADV_UNMERGEABLE- cancel MADV_MERGEABLE: no longer merge pages with others.
- *  MADV_HUGEPAGE - the application wants to back the given range by transparent
- *		huge pages in the future. Existing pages might be coalesced and
- *		new pages might be allocated as THP.
- *  MADV_NOHUGEPAGE - mark the given range as not worth being backed by
- *		transparent huge pages so the existing pages will not be
- *		coalesced into THP and new pages will not be allocated as THP.
- *  MADV_DONTDUMP - the application wants to prevent pages in the given range
- *		from being included in its core dump.
- *  MADV_DODUMP - cancel MADV_DONTDUMP: no longer exclude from core dump.
- *
- * return values:
- *  zero    - success
- *  -EINVAL - start + len < 0, start is not page-aligned,
- *		"behavior" is not a valid value, or application
- *		is attempting to release locked or shared pages,
- *		or the specified address range includes file, Huge TLB,
- *		MAP_SHARED or VMPFNMAP range.
- *  -ENOMEM - addresses in the specified range are not currently
- *		mapped, or are outside the AS of the process.
- *  -EIO    - an I/O error occurred while paging in data.
- *  -EBADF  - map exists, but area maps something that isn't a file.
- *  -EAGAIN - a kernel resource was temporarily unavailable.
- */
-SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
+static int madvise_core(struct task_struct *task, struct mm_struct *mm,
+			unsigned long start, size_t len_in, int behavior)

Just a minor nitpick, but can we please have it named madvise_common,
not madvise_core? This would follow a usual naming scheme, when some
common functionality is factored out (like, for mutexes, semaphores
etc), and within the kernel "core" usually means something completely
different.

Sure.

quoted

 {
 	unsigned long end, tmp;
 	struct vm_area_struct *vma, *prev;

@@ -1068,15 +1015,16 @@ SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
 
 #ifdef CONFIG_MEMORY_FAILURE
 	if (behavior == MADV_HWPOISON || behavior == MADV_SOFT_OFFLINE)
-		return madvise_inject_error(behavior, start, start + len_in);
+		return madvise_inject_error(behavior,
+					start, start + len_in);

Not sure what this change is about except changing the line length.
Note, madvise_inject_error() still operates on "current" through
get_user_pages_fast() and gup_pgd_range(), but that was not changed
here. I Know you've filtered out this hint later, so technically this
is not an issue, but, maybe, this needs some attention too since we've
already spotted it?

It is leftover I had done. I actually modified it to handle remote
task but changed my mind not to fix it because process_madvise
will not support it at this moment. I'm not sure it's a good idea
to change it for *might-be-done-in-future* at this moment even though
we have spotted.

quoted

 #endif
 
 	write = madvise_need_mmap_write(behavior);
 	if (write) {
-		if (down_write_killable(&current->mm->mmap_sem))
+		if (down_write_killable(&mm->mmap_sem))
 			return -EINTR;

Do you still need that trick with mmget_still_valid() here?
Something like:

Since MADV_COLD|PAGEOUT doesn't change address space layout or
vma->vm_flags, technically, we don't need it if I understand
correctly. Right?

if (current->mm != mm && !mmget_still_valid(mm))
   goto skip_mm;

and that skip_mm label would be before

if (write)
   up_write(&mm->mmap_sem);

below.

(see 04f5866e41fb70690e28397487d8bd8eea7d712a for details on this)