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Date:   Wed, 30 Nov 2022 08:28:03 -0800
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Shiyang Ruan <ruansy.fnst@fujitsu.com>
Cc:     linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org,
        nvdimm@lists.linux.dev, linux-fsdevel@vger.kernel.org,
        david@fromorbit.com, dan.j.williams@intel.com
Subject: Re: [PATCH 1/2] fsdax,xfs: fix warning messages at
 dax_[dis]associate_entry()
Message-ID: <Y4eEk8WLZHuj++za@magnolia>
References: <1669301694-16-1-git-send-email-ruansy.fnst@fujitsu.com>
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 <Y4bXTywl3PQTY3Er@magnolia>
 <7aa0a212-4cc8-04bc-10ea-4f0c47915623@fujitsu.com>
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On Wed, Nov 30, 2022 at 04:58:32PM +0800, Shiyang Ruan wrote:
> 
> 
> 在 2022/11/30 12:08, Darrick J. Wong 写道:
> > On Thu, Nov 24, 2022 at 02:54:53PM +0000, Shiyang Ruan wrote:
> > > This patch fixes the warning message reported in dax_associate_entry()
> > > and dax_disassociate_entry().
> > 
> > Hmm, that's quite a bit to put in a single patch, but I'll try to get
> > through this...
> 
> Oh sorry...

Well you have to start somewhere. :)

I often start with a megapatch for testing and later break it into
smaller pieces once I've validated that the megapatch creates a solid
improvement.

> > 
> > > 1. reset page->mapping and ->index when refcount counting down to 0.
> > > 2. set IOMAP_F_SHARED flag when iomap read to allow one dax page to be
> > > associated more than once for not only write but also read.
> > 
> > That makes sense, I think.
> > 
> > > 3. should zero the edge (when not aligned) if srcmap is HOLE or
> > 
> > When is IOMAP_F_SHARED set on the /source/ mapping?
> 
> In fs/xfs/xfs_iomap.c: xfs_direct_write_iomap_begin(): goto out_found_cow
> tag, srcmap is *not set* when the source extent is HOLE, then only iomap is
> set with IOMAP_F_SHARED flag.
> 
> Now we come to iomap iter, when we get the srcmap by calling
> iomap_iter_srcmap(iter), the iomap will be returned (because srcmap isn't
> set).  So, in this case, srcmap == iomap, we can think the source extent is
> a HOLE if srcmap->flag & IOMAP_F_SHARED != 0

Aha, got it.  IOWs, this handles things like alwayscow and cowing over a
hole, where we don't have a source mapping.  Thanks for refreshing my
memory.

> > > UNWRITTEN.
> > > 4. iterator of two files in dedupe should be executed side by side, not
> > > nested.
> > 
> > Why?  Also, this seems like a separate change?
> 
> Explain below.
> 
> > 
> > > 5. use xfs_dax_write_iomap_ops for xfs zero and truncate.
> > 
> > Makes sense.
> > 
> > > Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> > > ---
> > >   fs/dax.c           | 114 ++++++++++++++++++++++++++-------------------
> > >   fs/xfs/xfs_iomap.c |   6 +--
> > >   2 files changed, 69 insertions(+), 51 deletions(-)
> > > 
> > > diff --git a/fs/dax.c b/fs/dax.c
> > > index 1c6867810cbd..5ea7c0926b7f 100644
> > > --- a/fs/dax.c
> > > +++ b/fs/dax.c
> > > @@ -398,7 +398,7 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
> > >   		WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> > >   		if (dax_mapping_is_cow(page->mapping)) {
> > >   			/* keep the CoW flag if this page is still shared */
> > > -			if (page->index-- > 0)
> > > +			if (page->index-- > 1)
> > 
> > Hmm.  So if the fsdax "page" sharing factor drops from 2 to 1, we'll now
> > null out the mapping and index?  Before, we only did that when it
> > dropped from 1 to 0.
> > 
> > Does this leave the page with no mapping?  And I guess a subsequent
> > access will now take a fault to map it back in?
> 
> I confused it with --page->index, the result of "page->index--" is
> page->index itself.

Yeah, postfix operators in comparisons are not great for readability the
later one gets into the night.

> So, assume:
> this time, refcount is 2, >1,     minus 1 to 1, then continue;
> next time, refcount is 1, not >1, minus 1 to 0, then clear the
> page->mapping.

> 
> > 
> > >   				continue;
> > >   		} else
> > >   			WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> > > @@ -840,12 +840,6 @@ static bool dax_fault_is_synchronous(const struct iomap_iter *iter,
> > >   		(iter->iomap.flags & IOMAP_F_DIRTY);
> > >   }
> > > -static bool dax_fault_is_cow(const struct iomap_iter *iter)
> > > -{
> > > -	return (iter->flags & IOMAP_WRITE) &&
> > > -		(iter->iomap.flags & IOMAP_F_SHARED);
> > > -}
> > > -
> > >   /*
> > >    * By this point grab_mapping_entry() has ensured that we have a locked entry
> > >    * of the appropriate size so we don't have to worry about downgrading PMDs to
> > > @@ -859,13 +853,14 @@ static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
> > >   {
> > >   	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> > >   	void *new_entry = dax_make_entry(pfn, flags);
> > > -	bool dirty = !dax_fault_is_synchronous(iter, vmf->vma);
> > > -	bool cow = dax_fault_is_cow(iter);
> > > +	bool write = iter->flags & IOMAP_WRITE;
> > > +	bool dirty = write && !dax_fault_is_synchronous(iter, vmf->vma);
> > > +	bool shared = iter->iomap.flags & IOMAP_F_SHARED;
> > >   	if (dirty)
> > >   		__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
> > > -	if (cow || (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE))) {
> > > +	if (shared || (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE))) {
> > 
> > Ah, ok, so now we're yanking the mapping if the extent is shared,
> > presumably so that...
> > 
> > >   		unsigned long index = xas->xa_index;
> > >   		/* we are replacing a zero page with block mapping */
> > >   		if (dax_is_pmd_entry(entry))
> > > @@ -877,12 +872,12 @@ static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
> > >   	xas_reset(xas);
> > >   	xas_lock_irq(xas);
> > > -	if (cow || dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
> > > +	if (shared || dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
> > >   		void *old;
> > >   		dax_disassociate_entry(entry, mapping, false);
> > >   		dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
> > > -				cow);
> > > +				shared);
> > 
> > ...down here we can rebuild the association, but this time we'll set the
> > page->mapping to PAGE_MAPPING_DAX_COW?  I see a lot of similar changes,
> > so I'm guessing this is how you fixed the failures that were a result of
> > read file A -> reflink A to B -> read file B sequences?
> 
> Yes, it even failed when mapreading a page shared by two extent of ONE file.
> But I remember that I had tested these cases before...
> 
> > 
> > >   		/*
> > >   		 * Only swap our new entry into the page cache if the current
> > >   		 * entry is a zero page or an empty entry.  If a normal PTE or
> > > @@ -902,7 +897,7 @@ static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
> > >   	if (dirty)
> > >   		xas_set_mark(xas, PAGECACHE_TAG_DIRTY);
> > > -	if (cow)
> > > +	if (write && shared)
> > >   		xas_set_mark(xas, PAGECACHE_TAG_TOWRITE);
> > >   	xas_unlock_irq(xas);
> > > @@ -1107,23 +1102,35 @@ static int dax_iomap_cow_copy(loff_t pos, uint64_t length, size_t align_size,
> > 
> > I think this function isn't well named.  It's copying into the parts of
> > the @daddr page that are *not* covered by @pos/@length.  In other words,
> > it's really copying *around* the range that's supplied, isn't it?
> 
> Yes, I used to name it "dax_iomap_copy_edge()", which is not so good too.
> I'm not good at naming.

dax_iomap_cow_edges() ?

dax_iomap_copy_around() ?

> > >   	loff_t end = pos + length;
> > >   	loff_t pg_end = round_up(end, align_size);
> > >   	bool copy_all = head_off == 0 && end == pg_end;
> > > +	/* write zero at edge if srcmap is a HOLE or IOMAP_UNWRITTEN */
> > > +	bool zero_edge = srcmap->flags & IOMAP_F_SHARED ||
> > 
> > When is IOMAP_F_SHARED set on the /source/ mapping?  I don't understand
> > that circumstance, so I don't understand why we want to zero around in
> > that case.
> 
> Please see explanation above.
> 
> > 
> > > +			 srcmap->type == IOMAP_UNWRITTEN;
> > 
> > Though it's self evident why we'd do that if the source map is
> > unwritten.
> 
> The new allocated destination pages for CoW is not all zeroed, not like new
> allocated page cache.  Old data remains on it.  So, if the source extent
> doesn't contain valid data (HOLE and UNWRITTEN extent), we need to zero the
> destination range around the @pos+@length.

<nod>

> > 
> > >   	void *saddr = 0;
> > >   	int ret = 0;
> > > -	ret = dax_iomap_direct_access(srcmap, pos, size, &saddr, NULL);
> > > -	if (ret)
> > > -		return ret;
> > > +	if (!zero_edge) {
> > > +		ret = dax_iomap_direct_access(srcmap, pos, size, &saddr, NULL);
> > > +		if (ret)
> > > +			return ret;
> > > +	}
> > >   	if (copy_all) {
> > > -		ret = copy_mc_to_kernel(daddr, saddr, length);
> > > -		return ret ? -EIO : 0;
> > > +		if (zero_edge)
> > > +			memset(daddr, 0, size);
> > > +		else
> > > +			ret = copy_mc_to_kernel(daddr, saddr, length);
> > > +		goto out;
> > >   	}
> > >   	/* Copy the head part of the range */
> > >   	if (head_off) {
> > > -		ret = copy_mc_to_kernel(daddr, saddr, head_off);
> > > -		if (ret)
> > > -			return -EIO;
> > > +		if (zero_edge)
> > > +			memset(daddr, 0, head_off);
> > > +		else {
> > > +			ret = copy_mc_to_kernel(daddr, saddr, head_off);
> > > +			if (ret)
> > > +				return -EIO;
> > > +		}
> > >   	}
> > >   	/* Copy the tail part of the range */
> > > @@ -1131,12 +1138,19 @@ static int dax_iomap_cow_copy(loff_t pos, uint64_t length, size_t align_size,
> > >   		loff_t tail_off = head_off + length;
> > >   		loff_t tail_len = pg_end - end;
> > > -		ret = copy_mc_to_kernel(daddr + tail_off, saddr + tail_off,
> > > -					tail_len);
> > > -		if (ret)
> > > -			return -EIO;
> > > +		if (zero_edge)
> > > +			memset(daddr + tail_off, 0, tail_len);
> > > +		else {
> > > +			ret = copy_mc_to_kernel(daddr + tail_off,
> > > +						saddr + tail_off, tail_len);
> > > +			if (ret)
> > > +				return -EIO;
> > > +		}
> > >   	}
> > > -	return 0;
> > > +out:
> > > +	if (zero_edge)
> > > +		dax_flush(srcmap->dax_dev, daddr, size);
> > > +	return ret ? -EIO : 0;
> > >   }
> > >   /*
> > > @@ -1235,13 +1249,9 @@ static int dax_memzero(struct iomap_iter *iter, loff_t pos, size_t size)
> > >   	if (ret < 0)
> > >   		return ret;
> > >   	memset(kaddr + offset, 0, size);
> > > -	if (srcmap->addr != iomap->addr) {
> > > -		ret = dax_iomap_cow_copy(pos, size, PAGE_SIZE, srcmap,
> > > -					 kaddr);
> > > -		if (ret < 0)
> > > -			return ret;
> > > -		dax_flush(iomap->dax_dev, kaddr, PAGE_SIZE);
> > > -	} else
> > > +	if (iomap->flags & IOMAP_F_SHARED)
> > > +		ret = dax_iomap_cow_copy(pos, size, PAGE_SIZE, srcmap, kaddr);
> > > +	else
> > >   		dax_flush(iomap->dax_dev, kaddr + offset, size);
> > >   	return ret;
> > >   }
> > > @@ -1258,6 +1268,15 @@ static s64 dax_zero_iter(struct iomap_iter *iter, bool *did_zero)
> > >   	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
> > >   		return length;
> > > +	/*
> > > +	 * invalidate the pages whose sharing state is to be changed
> > > +	 * because of CoW.
> > > +	 */
> > > +	if (iomap->flags & IOMAP_F_SHARED)
> > > +		invalidate_inode_pages2_range(iter->inode->i_mapping,
> > > +					      pos >> PAGE_SHIFT,
> > > +					      (pos + length - 1) >> PAGE_SHIFT);
> > > +
> > >   	do {
> > >   		unsigned offset = offset_in_page(pos);
> > >   		unsigned size = min_t(u64, PAGE_SIZE - offset, length);
> > > @@ -1318,12 +1337,13 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
> > >   		struct iov_iter *iter)
> > >   {
> > >   	const struct iomap *iomap = &iomi->iomap;
> > > -	const struct iomap *srcmap = &iomi->srcmap;
> > > +	const struct iomap *srcmap = iomap_iter_srcmap(iomi);
> > >   	loff_t length = iomap_length(iomi);
> > >   	loff_t pos = iomi->pos;
> > >   	struct dax_device *dax_dev = iomap->dax_dev;
> > >   	loff_t end = pos + length, done = 0;
> > >   	bool write = iov_iter_rw(iter) == WRITE;
> > > +	bool cow = write && iomap->flags & IOMAP_F_SHARED;
> > >   	ssize_t ret = 0;
> > >   	size_t xfer;
> > >   	int id;
> > > @@ -1350,7 +1370,7 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
> > >   	 * into page tables. We have to tear down these mappings so that data
> > >   	 * written by write(2) is visible in mmap.
> > >   	 */
> > > -	if (iomap->flags & IOMAP_F_NEW) {
> > > +	if (iomap->flags & IOMAP_F_NEW || cow) {
> > >   		invalidate_inode_pages2_range(iomi->inode->i_mapping,
> > >   					      pos >> PAGE_SHIFT,
> > >   					      (end - 1) >> PAGE_SHIFT);
> > > @@ -1384,8 +1404,7 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
> > >   			break;
> > >   		}
> > > -		if (write &&
> > > -		    srcmap->type != IOMAP_HOLE && srcmap->addr != iomap->addr) {
> > > +		if (cow) {
> > >   			ret = dax_iomap_cow_copy(pos, length, PAGE_SIZE, srcmap,
> > >   						 kaddr);
> > >   			if (ret)
> > > @@ -1532,7 +1551,7 @@ static vm_fault_t dax_fault_iter(struct vm_fault *vmf,
> > >   		struct xa_state *xas, void **entry, bool pmd)
> > >   {
> > >   	const struct iomap *iomap = &iter->iomap;
> > > -	const struct iomap *srcmap = &iter->srcmap;
> > > +	const struct iomap *srcmap = iomap_iter_srcmap(iter);
> > >   	size_t size = pmd ? PMD_SIZE : PAGE_SIZE;
> > >   	loff_t pos = (loff_t)xas->xa_index << PAGE_SHIFT;
> > >   	bool write = iter->flags & IOMAP_WRITE;
> > > @@ -1563,8 +1582,7 @@ static vm_fault_t dax_fault_iter(struct vm_fault *vmf,
> > >   	*entry = dax_insert_entry(xas, vmf, iter, *entry, pfn, entry_flags);
> > > -	if (write &&
> > > -	    srcmap->type != IOMAP_HOLE && srcmap->addr != iomap->addr) {
> > > +	if (write && iomap->flags & IOMAP_F_SHARED) {
> > >   		err = dax_iomap_cow_copy(pos, size, size, srcmap, kaddr);
> > >   		if (err)
> > >   			return dax_fault_return(err);
> > > @@ -1936,15 +1954,15 @@ int dax_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
> > 
> > Does the dedupe change need to be in this patch?  It looks ok both
> > before and after, so I don't know why it's necessary.
> 
> I'll separate this in next version.
> 
> This is the old version:
> 	while ((ret = iomap_iter(&src_iter, ops)) > 0) {
> 		while ((ret = iomap_iter(&dst_iter, ops)) > 0) {
> 			dst_iter.processed = dax_range_compare_iter(&src_iter,
> 						&dst_iter, len, same);
> 		}
> 		if (ret <= 0)
> 			src_iter.processed = ret;
> 	}
> The inner iter (iomap_begin, actor, iomap_end) may loop more than once. In
> this case, the inner dest_iter updates its iomap, but the outer src_iter
> still keeps the old.  The comparison of new dest_iomap and old src_iomap is
> meanless and it causes the bug.

Ahh.  Got it.  The two iomap iters need to advance at the same time, or
the dst_iter needs to be defined in the outer loop body based on
whatever pos and length were returned by iomap_iter(&src_iter...).

> So, we need to make the two iters able to update their iomaps together.
> 
> > 
> > Welp, thank you for fixing the problems, at least.  After a couple of
> > days it looks like the serious problems have cleared up.
> > 
> 
> I didn't test the dax code well.  Sorry...

Well, it's still experimental code. :)

--D

> 
> --
> Thanks,
> Ruan.
> 
> > --D
> > 
> > >   		.len		= len,
> > >   		.flags		= IOMAP_DAX,
> > >   	};
> > > -	int ret;
> > > +	int ret, compared = 0;
> > > -	while ((ret = iomap_iter(&src_iter, ops)) > 0) {
> > > -		while ((ret = iomap_iter(&dst_iter, ops)) > 0) {
> > > -			dst_iter.processed = dax_range_compare_iter(&src_iter,
> > > -						&dst_iter, len, same);
> > > -		}
> > > -		if (ret <= 0)
> > > -			src_iter.processed = ret;
> > > +	while ((ret = iomap_iter(&src_iter, ops)) > 0 &&
> > > +	       (ret = iomap_iter(&dst_iter, ops)) > 0) {
> > > +		compared = dax_range_compare_iter(&src_iter, &dst_iter, len,
> > > +						  same);
> > > +		if (compared < 0)
> > > +			return ret;
> > > +		src_iter.processed = dst_iter.processed = compared;
> > >   	}
> > >   	return ret;
> > >   }
> > > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > > index 07da03976ec1..d9401d0300ad 100644
> > > --- a/fs/xfs/xfs_iomap.c
> > > +++ b/fs/xfs/xfs_iomap.c
> > > @@ -1215,7 +1215,7 @@ xfs_read_iomap_begin(
> > >   		return error;
> > >   	error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
> > >   			       &nimaps, 0);
> > > -	if (!error && (flags & IOMAP_REPORT))
> > > +	if (!error && ((flags & IOMAP_REPORT) || IS_DAX(inode)))
> > >   		error = xfs_reflink_trim_around_shared(ip, &imap, &shared);
> > >   	xfs_iunlock(ip, lockmode);
> > > @@ -1370,7 +1370,7 @@ xfs_zero_range(
> > >   	if (IS_DAX(inode))
> > >   		return dax_zero_range(inode, pos, len, did_zero,
> > > -				      &xfs_direct_write_iomap_ops);
> > > +				      &xfs_dax_write_iomap_ops);
> > >   	return iomap_zero_range(inode, pos, len, did_zero,
> > >   				&xfs_buffered_write_iomap_ops);
> > >   }
> > > @@ -1385,7 +1385,7 @@ xfs_truncate_page(
> > >   	if (IS_DAX(inode))
> > >   		return dax_truncate_page(inode, pos, did_zero,
> > > -					&xfs_direct_write_iomap_ops);
> > > +					&xfs_dax_write_iomap_ops);
> > >   	return iomap_truncate_page(inode, pos, did_zero,
> > >   				   &xfs_buffered_write_iomap_ops);
> > >   }
> > > -- 
> > > 2.38.1
> > >