Date:	Tue, 17 Jul 2012 14:16:48 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Lukáš Czerner <lczerner@...hat.com>
cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Dave Chinner <dchinner@...hat.com>, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, achender@...ux.vnet.ibm.com
Subject: Re: [PATCH 06/12 v2] mm: teach truncate_inode_pages_range() to hadnle
 non page aligned ranges

On Tue, 17 Jul 2012, Lukáš Czerner wrote:

> Date: Tue, 17 Jul 2012 13:57:42 +0200 (CEST)
> From: Lukáš Czerner <lczerner@...hat.com>
> To: Hugh Dickins <hughd@...gle.com>
> Cc: Lukas Czerner <lczerner@...hat.com>,
>     Andrew Morton <akpm@...ux-foundation.org>, Theodore Ts'o <tytso@....edu>,
>     Dave Chinner <dchinner@...hat.com>, linux-ext4@...r.kernel.org,
>     linux-fsdevel@...r.kernel.org, achender@...ux.vnet.ibm.com
> Subject: Re: [PATCH 06/12 v2] mm: teach truncate_inode_pages_range() to hadnle
>      non page aligned ranges
> 
> On Tue, 17 Jul 2012, Hugh Dickins wrote:
> 
> > Date: Tue, 17 Jul 2012 01:28:08 -0700 (PDT)
> > From: Hugh Dickins <hughd@...gle.com>
> > To: Lukas Czerner <lczerner@...hat.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>, Theodore Ts'o <tytso@....edu>,
> >     Dave Chinner <dchinner@...hat.com>, linux-ext4@...r.kernel.org,
> >     linux-fsdevel@...r.kernel.org, achender@...ux.vnet.ibm.com
> > Subject: Re: [PATCH 06/12 v2] mm: teach truncate_inode_pages_range() to hadnle
> >      non page aligned ranges
> > 
> > On Fri, 13 Jul 2012, Lukas Czerner wrote:
> > > This commit changes truncate_inode_pages_range() so it can handle non
> > > page aligned regions of the truncate. Currently we can hit BUG_ON when
> > > the end of the range is not page aligned, but he can handle unaligned
> > > start of the range.
> > > 
> > > Being able to handle non page aligned regions of the page can help file
> > > system punch_hole implementations and save some work, because once we're
> > > holding the page we might as well deal with it right away.
> > > 
> > > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > > Cc: Hugh Dickins <hughd@...gle.com>
> > 
> > As I said under 02/12, I'd much rather not change from the existing -1
> > convention: I don't think it's wonderful, but I do think it's confusing
> > and a waste of effort to change from it; and I'd rather keep the code
> > in truncate.c close to what's doing the same job in shmem.c.
> > 
> > Here's what I came up with (and hacked tmpfs to use it without swap
> > temporarily, so I could run fsx for an hour to validate it).  But you
> > can see I've a couple of questions; and probably ought to reduce the
> > partial page code duplication once we're sure what should go in there.
> > 
> > Hugh
> 
> Ok.
> 
> > 
> > [PATCH]...
> > 
> > Apply to truncate_inode_pages_range() the changes 83e4fa9c16e4 ("tmpfs:
> > support fallocate FALLOC_FL_PUNCH_HOLE") made to shmem_truncate_range():
> > so the generic function can handle partial end offset for hole-punching.
> > 
> > In doing tmpfs, I became convinced that it needed a set_page_dirty() on
> > the partial pages, and I'm doing that here: but perhaps it should be the
> > responsibility of the calling filesystem?  I don't know.
> 
> In file system, if the range is block aligned we do not need the page to
> be dirtied. However if it is not block aligned (at least in ext4)
> we're going to handle it ourselves and possibly mark the page buffer
> dirty (hence the page would be dirty). Also in case of data
> journalling, we'll have to take care of the last block in the hole
> ourselves. So I think file systems should take care of dirtying the
> partial page if needed.
> 
> > 
> > And I'm doubtful whether this code can be correct (on a filesystem with
> > blocksize less than pagesize) without adding an end offset argument to
> > address_space_operations invalidatepage(page, offset): convince me!
> 
> Well, I can't. It really seems that on block size < page size file
> systems we could potentially discard dirty buffers beyond the hole
> we're punching if it is not page aligned. We would probably need to
> add end offset argument to the invalidatepage() aop. However I do not
> seem to be able to trigger the problem yet so maybe I'm still
> missing something.

My bad, it definitely is not safe without the end offset argument in
invalidatepage() aops ..sigh..

> 
> -Lukas
> 
> > 
> > Not-yet-signed-off-by: Hugh Dickins <hughd@...gle.com>
> > ---
> > 
> >  mm/truncate.c |   69 +++++++++++++++++++++++++++++-------------------
> >  1 file changed, 42 insertions(+), 27 deletions(-)
> > 
> > --- 3.5-rc7/mm/truncate.c	2012-06-03 06:42:11.249787128 -0700
> > +++ linux/mm/truncate.c	2012-07-16 22:54:16.903821549 -0700
> > @@ -49,14 +49,6 @@ void do_invalidatepage(struct page *page
> >  		(*invalidatepage)(page, offset);
> >  }
> >  
> > -static inline void truncate_partial_page(struct page *page, unsigned partial)
> > -{
> > -	zero_user_segment(page, partial, PAGE_CACHE_SIZE);
> > -	cleancache_invalidate_page(page->mapping, page);
> > -	if (page_has_private(page))
> > -		do_invalidatepage(page, partial);
> > -}
> > -
> >  /*
> >   * This cancels just the dirty bit on the kernel page itself, it
> >   * does NOT actually remove dirty bits on any mmap's that may be
> > @@ -190,8 +182,8 @@ int invalidate_inode_page(struct page *p
> >   * @lend: offset to which to truncate
> >   *
> >   * Truncate the page cache, removing the pages that are between
> > - * specified offsets (and zeroing out partial page
> > - * (if lstart is not page aligned)).
> > + * specified offsets (and zeroing out partial pages
> > + * if lstart or lend + 1 is not page aligned).
> >   *
> >   * Truncate takes two passes - the first pass is nonblocking.  It will not
> >   * block on page locks and it will not block on writeback.  The second pass
> > @@ -206,31 +198,32 @@ int invalidate_inode_page(struct page *p
> >  void truncate_inode_pages_range(struct address_space *mapping,
> >  				loff_t lstart, loff_t lend)
> >  {
> > -	const pgoff_t start = (lstart + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
> > -	const unsigned partial = lstart & (PAGE_CACHE_SIZE - 1);
> > +	pgoff_t start = (lstart + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
> > +	pgoff_t end = (lend + 1) >> PAGE_CACHE_SHIFT;
> > +	unsigned int partial_start = lstart & (PAGE_CACHE_SIZE - 1);
> > +	unsigned int partial_end = (lend + 1) & (PAGE_CACHE_SIZE - 1);
> >  	struct pagevec pvec;
> >  	pgoff_t index;
> > -	pgoff_t end;
> >  	int i;
> >  
> >  	cleancache_invalidate_inode(mapping);
> >  	if (mapping->nrpages == 0)
> >  		return;
> >  
> > -	BUG_ON((lend & (PAGE_CACHE_SIZE - 1)) != (PAGE_CACHE_SIZE - 1));
> > -	end = (lend >> PAGE_CACHE_SHIFT);
> > +	if (lend == -1)
> > +		end = -1;	/* unsigned, so actually very big */
> >  
> >  	pagevec_init(&pvec, 0);
> >  	index = start;
> > -	while (index <= end && pagevec_lookup(&pvec, mapping, index,
> > -			min(end - index, (pgoff_t)PAGEVEC_SIZE - 1) + 1)) {
> > +	while (index < end && pagevec_lookup(&pvec, mapping, index,
> > +			min(end - index, (pgoff_t)PAGEVEC_SIZE))) {
> >  		mem_cgroup_uncharge_start();
> >  		for (i = 0; i < pagevec_count(&pvec); i++) {
> >  			struct page *page = pvec.pages[i];
> >  
> >  			/* We rely upon deletion not changing page->index */
> >  			index = page->index;
> > -			if (index > end)
> > +			if (index >= end)
> >  				break;
> >  
> >  			if (!trylock_page(page))
> > @@ -249,27 +242,51 @@ void truncate_inode_pages_range(struct a
> >  		index++;
> >  	}
> >  
> > -	if (partial) {
> > +	if (partial_start) {
> >  		struct page *page = find_lock_page(mapping, start - 1);
> >  		if (page) {
> > +			unsigned int top = PAGE_CACHE_SIZE;
> > +			if (start > end) {
> > +				top = partial_end;
> > +				partial_end = 0;
> > +			}
> >  			wait_on_page_writeback(page);
> > -			truncate_partial_page(page, partial);
> > +			zero_user_segment(page, partial_start, top);
> > +			cleancache_invalidate_page(mapping, page);
> > +			if (page_has_private(page))
> > +				do_invalidatepage(page, partial_start);
> > +			set_page_dirty(page);
> >  			unlock_page(page);
> >  			page_cache_release(page);
> >  		}
> >  	}
> > +	if (partial_end) {
> > +		struct page *page = find_lock_page(mapping, end);
> > +		if (page) {
> > +			wait_on_page_writeback(page);
> > +			zero_user_segment(page, 0, partial_end);
> > +			cleancache_invalidate_page(mapping, page);
> > +			if (page_has_private(page))
> > +				do_invalidatepage(page, 0);
> > +			set_page_dirty(page);
> > +			unlock_page(page);
> > +			page_cache_release(page);
> > +		}
> > +	}
> > +	if (start >= end)
> > +		return;
> >  
> >  	index = start;
> >  	for ( ; ; ) {
> >  		cond_resched();
> >  		if (!pagevec_lookup(&pvec, mapping, index,
> > -			min(end - index, (pgoff_t)PAGEVEC_SIZE - 1) + 1)) {
> > +			min(end - index, (pgoff_t)PAGEVEC_SIZE))) {
> >  			if (index == start)
> >  				break;
> >  			index = start;
> >  			continue;
> >  		}
> > -		if (index == start && pvec.pages[0]->index > end) {
> > +		if (index == start && pvec.pages[0]->index >= end) {
> >  			pagevec_release(&pvec);
> >  			break;
> >  		}
> > @@ -279,7 +296,7 @@ void truncate_inode_pages_range(struct a
> >  
> >  			/* We rely upon deletion not changing page->index */
> >  			index = page->index;
> > -			if (index > end)
> > +			if (index >= end)
> >  				break;
> >  
> >  			lock_page(page);
> > @@ -624,10 +641,8 @@ void truncate_pagecache_range(struct ino
> >  	 * This rounding is currently just for example: unmap_mapping_range
> >  	 * expands its hole outwards, whereas we want it to contract the hole
> >  	 * inwards.  However, existing callers of truncate_pagecache_range are
> > -	 * doing their own page rounding first; and truncate_inode_pages_range
> > -	 * currently BUGs if lend is not pagealigned-1 (it handles partial
> > -	 * page at start of hole, but not partial page at end of hole).  Note
> > -	 * unmap_mapping_range allows holelen 0 for all, and we allow lend -1.
> > +	 * doing their own page rounding first.  Note that unmap_mapping_range
> > +	 * allows holelen 0 for all, and we allow lend -1 for end of file.
> >  	 */
> >  
> >  	/*
> > 
> 

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