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Message-ID: <1484003510.2609.32.camel@redhat.com>
Date: Mon, 09 Jan 2017 18:11:50 -0500
From: Jeff Layton <jlayton@...hat.com>
To: Al Viro <viro@...IV.linux.org.uk>, "Yan, Zheng" <zyan@...hat.com>
Cc: Sage Weil <sage@...hat.com>, Ilya Dryomov <idryomov@...il.com>,
ceph-devel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org,
"Zhu, Caifeng" <zhucaifeng@...ssoft-nj.com>
Subject: Re: [PATCH] ceph/iov_iter: fix bad iov_iter handling in ceph splice
codepaths
On Fri, 2017-01-06 at 13:23 -0500, Jeff Layton wrote:
> xfstest generic/095 triggers soft lockups in kcephfs. Basically it uses
> fio to drive some I/O via vmsplice ane splice. Ceph then ends trying to
> access an ITER_BVEC type iov_iter as a ITER_IOVEC one. That causes it to
> pick up a wrong offset and get stuck in an infinite loop while trying to
> populate the page array.
>
> To fix this, add a new iov_iter helper for to determine the offset into
> the page for the current segment and have ceph call that.
>
> dio_get_pagev_size has similar problems, so fix that as well by adding a
> new helper function in lib/iov_iter.c that does what it does, but for
> all iov_iter types.
>
> Since we're moving that into generic code, we can also utilize the
> iterate_all_kinds macro to handle this. That means that we need to
> rework the logic a bit since we can't advance to the next vector while
> checking the current one.
>
> Link: http://tracker.ceph.com/issues/18130
> Cc: "Zhu, Caifeng" <zhucaifeng@...ssoft-nj.com>
> Signed-off-by: Jeff Layton <jlayton@...hat.com>
> ---
> fs/ceph/file.c | 28 ++------------------
> include/linux/uio.h | 2 ++
> lib/iov_iter.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 77 insertions(+), 26 deletions(-)
>
> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> index f633165f3fdc..0cd9fc68a04e 100644
> --- a/fs/ceph/file.c
> +++ b/fs/ceph/file.c
> @@ -35,29 +35,6 @@
> */
>
> /*
> - * Calculate the length sum of direct io vectors that can
> - * be combined into one page vector.
> - */
> -static size_t dio_get_pagev_size(const struct iov_iter *it)
> -{
> - const struct iovec *iov = it->iov;
> - const struct iovec *iovend = iov + it->nr_segs;
> - size_t size;
> -
> - size = iov->iov_len - it->iov_offset;
> - /*
> - * An iov can be page vectored when both the current tail
> - * and the next base are page aligned.
> - */
> - while (PAGE_ALIGNED((iov->iov_base + iov->iov_len)) &&
> - (++iov < iovend && PAGE_ALIGNED((iov->iov_base)))) {
> - size += iov->iov_len;
> - }
> - dout("dio_get_pagevlen len = %zu\n", size);
> - return size;
> -}
> -
> -/*
> * Allocate a page vector based on (@it, @nbytes).
> * The return value is the tuple describing a page vector,
> * that is (@pages, @page_align, @num_pages).
> @@ -71,8 +48,7 @@ dio_get_pages_alloc(const struct iov_iter *it, size_t nbytes,
> struct page **pages;
> int ret = 0, idx, npages;
>
> - align = (unsigned long)(it->iov->iov_base + it->iov_offset) &
> - (PAGE_SIZE - 1);
> + align = iov_iter_single_seg_page_offset(it);
> npages = calc_pages_for(align, nbytes);
> pages = kmalloc(sizeof(*pages) * npages, GFP_KERNEL);
> if (!pages) {
Found the bug with the earlier patch, and will be sending an updated one
after some more testing.
That said, I would love to be able to just get rid of ceph's
dio_get_pages_alloc as well and just use iov_iter_get_pages_alloc
instead, but I don't think we can because the bio_vec iterator block
does this:
/* can't be more than PAGE_SIZE */
*start = v.bv_offset;
*pages = p = get_pages_array(1);
if (!p)
return -ENOMEM;
get_page(*p = v.bv_page);
return v.bv_len;
...so what happens here is that iter_file_splice_write creates an
ITER_BVEC iov_iter. Then any filesystem driver that uses
iov_iter_get_pages or iov_iter_get_pages_alloc can only get a single
page for each call. The ceph code works around this by calling
iov_iter_get_pages in a loop until it gets enough, but that's rather
nasty.
I bet this makes splice writing to an O_DIRECT file on NFS suck too...
Why _is_ ITER_BVEC limited to a single page there? I wonder if we could
consider lifting that limitation in some cases if it really is
necessary?
> @@ -927,7 +903,7 @@ ceph_direct_read_write(struct kiocb *iocb, struct iov_iter *iter,
> }
>
> while (iov_iter_count(iter) > 0) {
> - u64 size = dio_get_pagev_size(iter);
> + u64 size = iov_iter_pvec_size(iter);
> size_t start = 0;
> ssize_t len;
>
>
> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index 6e22b544d039..46fdfff3d7d6 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
> @@ -83,6 +83,7 @@ size_t iov_iter_copy_from_user_atomic(struct page *page,
> void iov_iter_advance(struct iov_iter *i, size_t bytes);
> int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes);
> size_t iov_iter_single_seg_count(const struct iov_iter *i);
> +size_t iov_iter_single_seg_page_offset(const struct iov_iter *i);
> size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
> struct iov_iter *i);
> size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
> @@ -93,6 +94,7 @@ size_t copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i);
> size_t iov_iter_zero(size_t bytes, struct iov_iter *);
> unsigned long iov_iter_alignment(const struct iov_iter *i);
> unsigned long iov_iter_gap_alignment(const struct iov_iter *i);
> +size_t iov_iter_pvec_size(const struct iov_iter *i);
> void iov_iter_init(struct iov_iter *i, int direction, const struct iovec *iov,
> unsigned long nr_segs, size_t count);
> void iov_iter_kvec(struct iov_iter *i, int direction, const struct kvec *kvec,
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 6b415b5a100d..d22850bbb1e9 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -745,6 +745,24 @@ size_t iov_iter_single_seg_count(const struct iov_iter *i)
> }
> EXPORT_SYMBOL(iov_iter_single_seg_count);
>
> +/* Return offset into page for current iov_iter segment */
> +size_t iov_iter_single_seg_page_offset(const struct iov_iter *i)
> +{
> + size_t base;
> +
> + if (i->type & ITER_PIPE)
> + base = i->pipe->bufs[i->idx].offset;
> + else if (i->type & ITER_BVEC)
> + base = i->bvec->bv_offset;
> + else if (i->type & ITER_KVEC)
> + base = (size_t)i->kvec->iov_base;
> + else
> + base = (size_t)i->iov->iov_base;
> +
> + return (base + i->iov_offset) & (PAGE_SIZE - 1);
> +}
> +EXPORT_SYMBOL(iov_iter_single_seg_page_offset);
> +
> void iov_iter_kvec(struct iov_iter *i, int direction,
> const struct kvec *kvec, unsigned long nr_segs,
> size_t count)
> @@ -830,6 +848,61 @@ unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
> }
> EXPORT_SYMBOL(iov_iter_gap_alignment);
>
> +/**
> + * iov_iter_pvec_size - find length of page aligned iovecs in iov_iter
> + * @i: iov_iter to in which to find the size
> + *
> + * Some filesystems can stitch together multiple iovecs into a single
> + * page vector when both the previous tail and current base are page
> + * aligned. This function discovers the length that can fit in a single
> + * pagevec and returns it.
> + */
> +size_t iov_iter_pvec_size(const struct iov_iter *i)
> +{
> + size_t size = i->count;
> + size_t pv_size = 0;
> + bool contig = false, first = true;
> +
> + if (!size)
> + return 0;
> +
> + /* Pipes are naturally aligned for this */
> + if (unlikely(i->type & ITER_PIPE))
> + return size;
> +
> + /*
> + * An iov can be page vectored when the current base and previous
> + * tail are both page aligned. Note that we don't require that the
> + * initial base in the first iovec also be page aligned.
> + */
> + iterate_all_kinds(i, size, v,
> + ({
> + if (first || (contig && PAGE_ALIGNED(v.iov_base))) {
> + pv_size += v.iov_len;
> + first = false;
> + contig = PAGE_ALIGNED(v.iov_base + v.iov_len);
> + }; 0;
> + }),
> + ({
> + if (first || (contig && v.bv_offset == 0)) {
> + pv_size += v.bv_len;
> + first = false;
> + contig = PAGE_ALIGNED(v.bv_offset + v.bv_len);
> + }
> + }),
> + ({
> + if (first || (contig && PAGE_ALIGNED(v.iov_base))) {
> + pv_size += v.iov_len;
> + first = false;
> + contig = PAGE_ALIGNED(v.iov_base + v.iov_len);
> + }
> + }))
> +
> + pv_size -= i->iov_offset;
The bug in this patch is the above. iterate_all_kinds will have already
accounted for the iov_offset, so there's no need to do this here (and it
throws off the calculation when you have to go around more than once).
> + return pv_size;
> +}
> +EXPORT_SYMBOL(iov_iter_pvec_size);
> +
> static inline size_t __pipe_get_pages(struct iov_iter *i,
> size_t maxsize,
> struct page **pages,
--
Jeff Layton <jlayton@...hat.com>
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