On Tue, Dec 17, 2019 at 07:39:47AM -0700, Jens Axboe wrote:
> This adds support for RWF_UNCACHED for file systems using iomap to
> perform buffered writes. We use the generic infrastructure for this,
> by tracking pages we created and calling write_drop_cached_pages()
> to issue writeback and prune those pages.
>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> ---
> fs/iomap/apply.c | 35 +++++++++++++++++++++++++++++++++++
> fs/iomap/buffered-io.c | 28 ++++++++++++++++++++++++----
> fs/iomap/trace.h | 4 +++-
> include/linux/iomap.h | 5 +++++
> 4 files changed, 67 insertions(+), 5 deletions(-)
>
> diff --git a/fs/iomap/apply.c b/fs/iomap/apply.c
> index 792079403a22..687e86945b27 100644
> --- a/fs/iomap/apply.c
> +++ b/fs/iomap/apply.c
> @@ -92,5 +92,40 @@ iomap_apply(struct iomap_ctx *data, const struct iomap_ops *ops,
> data->flags, &iomap);
> }
>
> + if (written <= 0)
> + goto out;
> +
> + /*
> + * If this is an uncached write, then we need to write and sync this
> + * range of data. This is only true for a buffered write, not for
> + * O_DIRECT.
> + */
I tracked down the original conversation, where Dave had this to say:
"Hence, IMO, this is the wrong layer in iomap to be dealing with¬
writeback and cache residency for uncached IO. We should be caching¬
residency/invalidation at a per-IO level, not a per-page level."
He's right, but I still think it doesn't quite smell right to be putting
this in iomap_apply, since that's a generic function that implements
iteration and shouldn't be messing with cache invalidation.
So I have two possible suggestions for where to put this:
(1) Add the "flush and maybe invalidate" behavior to the bottom of
iomap_write_actor like I said in the v4 patchset. That will issue
writeback and invalidate pagecache in smallish quantities.
(2) Find a way to pass the IOMAP_F_PAGE_CREATE state from
iomap_write_actor back to iomap_file_buffered_write and do the
flush-and-invalidate for the entire write request once at the end.
> + if ((data->flags & (IOMAP_WRITE|IOMAP_DIRECT|IOMAP_UNCACHED)) ==
> + (IOMAP_WRITE|IOMAP_UNCACHED)) {
> + struct address_space *mapping = data->inode->i_mapping;
> +
> + end = data->pos + written;
> + ret = filemap_write_and_wait_range(mapping, data->pos, end);
> + if (ret)
> + goto out;
> +
> + /*
> + * No pages were created for this range, we're done. We only
> + * invalidate the range if no pages were created for the
> + * entire range.
> + */
> + if (!(iomap.flags & IOMAP_F_PAGE_CREATE))
> + goto out;
> +
> + /*
> + * Try to invalidate cache pages for the range we just wrote.
> + * We don't care if invalidation fails as the write has still
> + * worked and leaving clean uptodate pages in the page cache
> + * isn't a corruption vector for uncached IO.
> + */
> + invalidate_inode_pages2_range(mapping,
> + data->pos >> PAGE_SHIFT, end >> PAGE_SHIFT);
> + }
> +out:
> return written ? written : ret;
> }
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 7f8300bce767..328afeba950f 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -582,6 +582,7 @@ EXPORT_SYMBOL_GPL(iomap_migrate_page);
>
> enum {
> IOMAP_WRITE_F_UNSHARE = (1 << 0),
> + IOMAP_WRITE_F_UNCACHED = (1 << 1),
> };
>
> static void
> @@ -659,6 +660,7 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> struct page **pagep, struct iomap *iomap, struct iomap *srcmap)
> {
> const struct iomap_page_ops *page_ops = iomap->page_ops;
> + unsigned aop_flags;
> struct page *page;
> int status = 0;
>
> @@ -675,8 +677,11 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> return status;
> }
>
> + aop_flags = AOP_FLAG_NOFS;
> + if (flags & IOMAP_WRITE_F_UNCACHED)
> + aop_flags |= AOP_FLAG_UNCACHED;
> page = grab_cache_page_write_begin(inode->i_mapping, pos >> PAGE_SHIFT,
> - AOP_FLAG_NOFS);
> + aop_flags);
> if (!page) {
> status = -ENOMEM;
> goto out_no_page;
> @@ -820,9 +825,13 @@ iomap_write_actor(const struct iomap_ctx *data, struct iomap *iomap,
> struct iov_iter *i = data->priv;
> loff_t length = data->len;
> loff_t pos = data->pos;
> + unsigned flags = 0;
> long status = 0;
> ssize_t written = 0;
>
> + if (data->flags & IOMAP_UNCACHED)
> + flags |= IOMAP_WRITE_F_UNCACHED;
> +
> do {
> struct page *page;
> unsigned long offset; /* Offset into pagecache page */
> @@ -851,10 +860,18 @@ iomap_write_actor(const struct iomap_ctx *data, struct iomap *iomap,
> break;
> }
>
> - status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap,
> - srcmap);
> - if (unlikely(status))
> +retry:
> + status = iomap_write_begin(inode, pos, bytes, flags,
> + &page, iomap, srcmap);
> + if (unlikely(status)) {
> + if (status == -ENOMEM &&
> + (flags & IOMAP_WRITE_F_UNCACHED)) {
> + iomap->flags |= IOMAP_F_PAGE_CREATE;
> + flags &= ~IOMAP_WRITE_F_UNCACHED;
What's the strategy here? We couldn't get a page for an uncached write,
so try again as a regular cached write?
Thanks for making the updates, it's looking better.
--D
> + goto retry;
> + }
> break;
> + }
>
> if (mapping_writably_mapped(inode->i_mapping))
> flush_dcache_page(page);
> @@ -907,6 +924,9 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *iter,
> };
> loff_t ret = 0, written = 0;
>
> + if (iocb->ki_flags & IOCB_UNCACHED)
> + data.flags |= IOMAP_UNCACHED;
> +
> while (iov_iter_count(iter)) {
> data.len = iov_iter_count(iter);
> ret = iomap_apply(&data, ops, iomap_write_actor);
> diff --git a/fs/iomap/trace.h b/fs/iomap/trace.h
> index 6dc227b8c47e..63c771e3eef5 100644
> --- a/fs/iomap/trace.h
> +++ b/fs/iomap/trace.h
> @@ -93,7 +93,8 @@ DEFINE_PAGE_EVENT(iomap_invalidatepage);
> { IOMAP_REPORT, "REPORT" }, \
> { IOMAP_FAULT, "FAULT" }, \
> { IOMAP_DIRECT, "DIRECT" }, \
> - { IOMAP_NOWAIT, "NOWAIT" }
> + { IOMAP_NOWAIT, "NOWAIT" }, \
> + { IOMAP_UNCACHED, "UNCACHED" }
>
> #define IOMAP_F_FLAGS_STRINGS \
> { IOMAP_F_NEW, "NEW" }, \
> @@ -101,6 +102,7 @@ DEFINE_PAGE_EVENT(iomap_invalidatepage);
> { IOMAP_F_SHARED, "SHARED" }, \
> { IOMAP_F_MERGED, "MERGED" }, \
> { IOMAP_F_BUFFER_HEAD, "BH" }, \
> + { IOMAP_F_PAGE_CREATE, "PAGE_CREATE" }, \
> { IOMAP_F_SIZE_CHANGED, "SIZE_CHANGED" }
>
> DECLARE_EVENT_CLASS(iomap_class,
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 00e439aac8ea..58311b6fdfdd 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -48,12 +48,16 @@ struct vm_fault;
> *
> * IOMAP_F_BUFFER_HEAD indicates that the file system requires the use of
> * buffer heads for this mapping.
> + *
> + * IOMAP_F_PAGE_CREATE indicates that pages had to be allocated to satisfy
> + * this operation.
> */
> #define IOMAP_F_NEW 0x01
> #define IOMAP_F_DIRTY 0x02
> #define IOMAP_F_SHARED 0x04
> #define IOMAP_F_MERGED 0x08
> #define IOMAP_F_BUFFER_HEAD 0x10
> +#define IOMAP_F_PAGE_CREATE 0x20
>
> /*
> * Flags set by the core iomap code during operations:
> @@ -121,6 +125,7 @@ struct iomap_page_ops {
> #define IOMAP_FAULT (1 << 3) /* mapping for page fault */
> #define IOMAP_DIRECT (1 << 4) /* direct I/O */
> #define IOMAP_NOWAIT (1 << 5) /* do not block */
> +#define IOMAP_UNCACHED (1 << 6) /* uncached IO */
>
> struct iomap_ops {
> /*
> --
> 2.24.1
>
