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

[Minchan Kim]

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:
> > 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@xxxxxxxxxx>
> > ---
> >  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.

> 
> >  {
> >  	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.

> 
> >  #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)