RE: [PATCH v4 1/7] fsdax: Introduce dax_iomap_cow_copy()

[ruansy.fnst@xxxxxxxxxxx]

> -----Original Message-----
> From: Darrick J. Wong <djwong@xxxxxxxxxx>
> Sent: Friday, April 9, 2021 5:53 AM
> Subject: Re: [PATCH v4 1/7] fsdax: Introduce dax_iomap_cow_copy()
> 
> On Thu, Apr 08, 2021 at 08:04:26PM +0800, Shiyang Ruan wrote:
> > In the case where the iomap is a write operation and iomap is not
> > equal to srcmap after iomap_begin, we consider it is a CoW operation.
> >
> > The destance extent which iomap indicated is new allocated extent.
> > So, it is needed to copy the data from srcmap to new allocated extent.
> > In theory, it is better to copy the head and tail ranges which is
> > outside of the non-aligned area instead of copying the whole aligned
> > range. But in dax page fault, it will always be an aligned range.  So,
> > we have to copy the whole range in this case.
> >
> > Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
> > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > ---
> >  fs/dax.c | 82
> > ++++++++++++++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 77 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 8d7e4e2cc0fb..b4fd3813457a 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -1038,6 +1038,61 @@ static int dax_iomap_direct_access(struct iomap
> *iomap, loff_t pos, size_t size,
> >  	return rc;
> >  }
> >
> > +/**
> > + * dax_iomap_cow_copy(): Copy the data from source to destination before
> write.
> > + * @pos:	address to do copy from.
> > + * @length:	size of copy operation.
> > + * @align_size:	aligned w.r.t align_size (either PMD_SIZE or PAGE_SIZE)
> > + * @srcmap:	iomap srcmap
> > + * @daddr:	destination address to copy to.
> > + *
> > + * This can be called from two places. Either during DAX write fault,
> > +to copy
> > + * the length size data to daddr. Or, while doing normal DAX write
> > +operation,
> > + * dax_iomap_actor() might call this to do the copy of either start
> > +or end
> > + * unaligned address. In this case the rest of the copy of aligned
> > +ranges is
> > + * taken care by dax_iomap_actor() itself.
> 
> Er... what?  This description is very confusing to me.  /me reads the code,
> and ...
> 
> OH.
> 
> Given a range (pos, length) and a mapping for a source file, this function copies
> all the bytes between pos and (pos + length) to daddr if the range is aligned to
> @align_size.  But if pos and length are not both aligned to align_src then it'll
> copy *around* the range, leaving the area in the middle uncopied waiting for
> write_iter to fill it in with whatever's in the iovec.
> 
> Yikes, this function is doing double duty and ought to be split into two functions.
> 
> The first function does the COW work for a write fault to an mmap region and
> does a straight copy.  Page faults are always aligned, so this functionality is
> needed by dax_fault_actor.  Maybe this could be named dax_fault_cow?
> 
> The second function does the prep COW work *around* a write so that we
> always copy entire page/blocks.  This cow-around code is needed by
> dax_iomap_actor.  This should perhaps be named dax_iomap_cow_around()?

Two functions seems easier to understand.  But I think the code from dax_iomap_direct_access() to its above will be redundant in this two functions.
How about make the description better?

> 
> > + * Also, note DAX fault will always result in aligned pos and pos + length.
> > + */
> > +static int dax_iomap_cow_copy(loff_t pos, loff_t length, size_t align_size,
> > +		struct iomap *srcmap, void *daddr)
> > +{
> > +	loff_t head_off = pos & (align_size - 1);
> > +	size_t size = ALIGN(head_off + length, align_size);
> > +	loff_t end = pos + length;
> > +	loff_t pg_end = round_up(end, align_size);
> > +	bool copy_all = head_off == 0 && end == pg_end;
> > +	void *saddr = 0;
> > +	int ret = 0;
> > +
> > +	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;
> 
> I find it /very/ interesting that copy_mc_to_kernel takes an unsigned int
> argument but returns an unsigned long (counting the bytes that didn't get
> copied, oddly...but that's an existing API so I guess I'll let it go.)
> 
> > +	}
> > +
> > +	/* Copy the head part of the range.  Note: we pass offset as length. */
> > +	if (head_off) {
> > +		ret = copy_mc_to_kernel(daddr, saddr, head_off);
> > +		if (ret)
> > +			return -EIO;
> > +	}
> > +
> > +	/* Copy the tail part of the range */
> > +	if (end < pg_end) {
> > +		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;
> > +	}
> > +	return 0;
> > +}
> > +
> >  /*
> >   * The user has performed a load from a hole in the file.  Allocating a new
> >   * page in the file would cause excessive storage usage for workloads
> > with @@ -1167,11 +1222,12 @@ dax_iomap_actor(struct inode *inode,
> loff_t pos, loff_t length, void *data,
> >  	struct dax_device *dax_dev = iomap->dax_dev;
> >  	struct iov_iter *iter = data;
> >  	loff_t end = pos + length, done = 0;
> > +	bool write = iov_iter_rw(iter) == WRITE;
> >  	ssize_t ret = 0;
> >  	size_t xfer;
> >  	int id;
> >
> > -	if (iov_iter_rw(iter) == READ) {
> > +	if (!write) {
> >  		end = min(end, i_size_read(inode));
> >  		if (pos >= end)
> >  			return 0;
> > @@ -1180,7 +1236,8 @@ dax_iomap_actor(struct inode *inode, loff_t pos,
> loff_t length, void *data,
> >  			return iov_iter_zero(min(length, end - pos), iter);
> >  	}
> >
> > -	if (WARN_ON_ONCE(iomap->type != IOMAP_MAPPED))
> > +	if (WARN_ON_ONCE(iomap->type != IOMAP_MAPPED &&
> > +			!(iomap->flags & IOMAP_F_SHARED)))
> 
> This is a bit subtle.  Could we add a comment:
> 
> 	/*
> 	 * In DAX mode, we allow either pure overwrites of written extents,
> 	 * or writes to unwritten extents as part of a copy-on-write
> 	 * operation.
> 	 */
> 	if (WARN_ON_ONCE(...))

OK.

> 
> >  		return -EIO;
> >
> >  	/*
> > @@ -1219,6 +1276,13 @@ dax_iomap_actor(struct inode *inode, loff_t pos,
> loff_t length, void *data,
> >  			break;
> >  		}
> >
> > +		if (write && srcmap->addr != iomap->addr) {
> 
> Do you have to check if srcmap is not a hole?
This dax_iomap_actor() is called by iomap_apply(), in which srcmap has been checked: If srcmap is a HOLE, then iomap_apply() will tell the actor that iomap == srcmap.  So, I didn't check it here.  But in dax_fault_actor() case, because we are not using iomap_apply(), the check is needed.


--
Thanks,
Ruan Shiyang.
> 
> --D
> 
> > +			ret = dax_iomap_cow_copy(pos, length, PAGE_SIZE, srcmap,
> > +						 kaddr);
> > +			if (ret)
> > +				break;
> > +		}
> > +
> >  		map_len = PFN_PHYS(map_len);
> >  		kaddr += offset;
> >  		map_len -= offset;
> > @@ -1230,7 +1294,7 @@ dax_iomap_actor(struct inode *inode, loff_t pos,
> loff_t length, void *data,
> >  		 * validated via access_ok() in either vfs_read() or
> >  		 * vfs_write(), depending on which operation we are doing.
> >  		 */
> > -		if (iov_iter_rw(iter) == WRITE)
> > +		if (write)
> >  			xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr,
> >  					map_len, iter);
> >  		else
> > @@ -1382,6 +1446,7 @@ static vm_fault_t dax_fault_actor(struct vm_fault
> *vmf, pfn_t *pfnp,
> >  	unsigned long entry_flags = pmd ? DAX_PMD : 0;
> >  	int err = 0;
> >  	pfn_t pfn;
> > +	void *kaddr;
> >
> >  	/* if we are reading UNWRITTEN and HOLE, return a hole. */
> >  	if (!write &&
> > @@ -1392,18 +1457,25 @@ static vm_fault_t dax_fault_actor(struct
> vm_fault *vmf, pfn_t *pfnp,
> >  			return dax_pmd_load_hole(xas, vmf, iomap, entry);
> >  	}
> >
> > -	if (iomap->type != IOMAP_MAPPED) {
> > +	if (iomap->type != IOMAP_MAPPED && !(iomap->flags &
> IOMAP_F_SHARED))
> > +{
> >  		WARN_ON_ONCE(1);
> >  		return pmd ? VM_FAULT_FALLBACK : VM_FAULT_SIGBUS;
> >  	}
> >
> > -	err = dax_iomap_direct_access(iomap, pos, size, NULL, &pfn);
> > +	err = dax_iomap_direct_access(iomap, pos, size, &kaddr, &pfn);
> >  	if (err)
> >  		return pmd ? VM_FAULT_FALLBACK : dax_fault_return(err);
> >
> >  	*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn, entry_flags,
> >  				  write && !sync);
> >
> > +	if (write &&
> > +	    srcmap->addr != IOMAP_HOLE && srcmap->addr != iomap->addr) {
> > +		err = dax_iomap_cow_copy(pos, size, size, srcmap, kaddr);
> > +		if (err)
> > +			return dax_fault_return(err);
> > +	}
> > +
> >  	if (sync)
> >  		return dax_fault_synchronous_pfnp(pfnp, pfn);
> >
> > --
> > 2.31.0
> >
> >
> >