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Date: Tue, 11 May 2021 18:08:10 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc: linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
linux-nvdimm@...ts.01.org, linux-fsdevel@...r.kernel.org,
darrick.wong@...cle.com, dan.j.williams@...el.com,
willy@...radead.org, viro@...iv.linux.org.uk, david@...morbit.com,
hch@....de, rgoldwyn@...e.de
Subject: Re: [PATCH v5 1/7] fsdax: Introduce dax_iomap_cow_copy()
On Tue, May 11, 2021 at 11:09:27AM +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@...itsu.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> ---
> fs/dax.c | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
> 1 file changed, 81 insertions(+), 5 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index bf3fc8242e6c..f0249bb1d46a 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.
> + * 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,
Nit: Linus has asked us not to continue the use of loff_t for file
io length. Could you change this to 'uint64_t length', please?
(Assuming we even need the extra length bits?)
With that fixed up...
Reviewed-by: Darrick J. Wong <djwong@...nel.org>
--D
> + struct iomap *srcmap, void *daddr)
> +{
> + loff_t head_off = pos & (align_size - 1);
Other nit: head_off = round_down(pos, align_size); ?
> + 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;
> + }
> +
> + /* 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 +1236,12 @@ 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))
> + /*
> + * 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(iomap->type != IOMAP_MAPPED &&
> + !(iomap->flags & IOMAP_F_SHARED)))
> return -EIO;
>
> /*
> @@ -1219,6 +1280,13 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> break;
> }
>
> + if (write && srcmap->addr != iomap->addr) {
> + 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 +1298,7 @@ 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 +1450,7 @@ 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 +1461,25 @@ 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);
>
> --
> 2.31.1
>
>
>
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