130 lines
5.3 KiB
Diff
130 lines
5.3 KiB
Diff
From 790cebc2a1d8de8d93b2a2a0ef19e31c767f4f1c Mon Sep 17 00:00:00 2001
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From: Max Reitz <mreitz@redhat.com>
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Date: Mon, 9 Sep 2019 07:38:20 +0100
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Subject: [PATCH 4/6] file-posix: Handle undetectable alignment
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RH-Author: Max Reitz <mreitz@redhat.com>
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Message-id: <20190909073822.26191-2-mreitz@redhat.com>
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Patchwork-id: 90332
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O-Subject: [RHEL-AV-8.1.0 qemu-kvm PATCH 1/3] file-posix: Handle undetectable alignment
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Bugzilla: 1749134
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RH-Acked-by: David Hildenbrand <david@redhat.com>
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RH-Acked-by: Thomas Huth <thuth@redhat.com>
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RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
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From: Nir Soffer <nirsof@gmail.com>
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In some cases buf_align or request_alignment cannot be detected:
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1. With Gluster, buf_align cannot be detected since the actual I/O is
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done on Gluster server, and qemu buffer alignment does not matter.
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Since we don't have alignment requirement, buf_align=1 is the best
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value.
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2. With local XFS filesystem, buf_align cannot be detected if reading
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from unallocated area. In this we must align the buffer, but we don't
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know what is the correct size. Using the wrong alignment results in
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I/O error.
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3. With Gluster backed by XFS, request_alignment cannot be detected if
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reading from unallocated area. In this case we need to use the
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correct alignment, and failing to do so results in I/O errors.
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4. With NFS, the server does not use direct I/O, so both buf_align cannot
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be detected. In this case we don't need any alignment so we can use
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buf_align=1 and request_alignment=1.
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These cases seems to work when storage sector size is 512 bytes, because
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the current code starts checking align=512. If the check succeeds
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because alignment cannot be detected we use 512. But this does not work
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for storage with 4k sector size.
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To determine if we can detect the alignment, we probe first with
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align=1. If probing succeeds, maybe there are no alignment requirement
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(cases 1, 4) or we are probing unallocated area (cases 2, 3). Since we
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don't have any way to tell, we treat this as undetectable alignment. If
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probing with align=1 fails with EINVAL, but probing with one of the
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expected alignments succeeds, we know that we found a working alignment.
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Practically the alignment requirements are the same for buffer
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alignment, buffer length, and offset in file. So in case we cannot
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detect buf_align, we can use request alignment. If we cannot detect
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request alignment, we can fallback to a safe value. To use this logic,
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we probe first request alignment instead of buf_align.
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Here is a table showing the behaviour with current code (the value in
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parenthesis is the optimal value).
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Case Sector buf_align (opt) request_alignment (opt) result
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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block/file-posix.c | 36 +++++++++++++++++++++++++-----------
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1 file changed, 25 insertions(+), 11 deletions(-)
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diff --git a/block/file-posix.c b/block/file-posix.c
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index 4479cc7..b8b4dad 100644
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--- a/block/file-posix.c
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+++ b/block/file-posix.c
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@@ -323,6 +323,7 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
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BDRVRawState *s = bs->opaque;
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char *buf;
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size_t max_align = MAX(MAX_BLOCKSIZE, getpagesize());
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+ size_t alignments[] = {1, 512, 1024, 2048, 4096};
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/* For SCSI generic devices the alignment is not really used.
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With buffered I/O, we don't have any restrictions. */
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@@ -349,25 +350,38 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
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}
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#endif
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- /* If we could not get the sizes so far, we can only guess them */
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- if (!s->buf_align) {
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+ /*
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+ * If we could not get the sizes so far, we can only guess them. First try
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+ * to detect request alignment, since it is more likely to succeed. Then
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+ * try to detect buf_align, which cannot be detected in some cases (e.g.
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+ * Gluster). If buf_align cannot be detected, we fallback to the value of
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+ * request_alignment.
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+ */
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+
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+ if (!bs->bl.request_alignment) {
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+ int i;
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size_t align;
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- buf = qemu_memalign(max_align, 2 * max_align);
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- for (align = 512; align <= max_align; align <<= 1) {
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- if (raw_is_io_aligned(fd, buf + align, max_align)) {
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- s->buf_align = align;
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+ buf = qemu_memalign(max_align, max_align);
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+ for (i = 0; i < ARRAY_SIZE(alignments); i++) {
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+ align = alignments[i];
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+ if (raw_is_io_aligned(fd, buf, align)) {
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+ /* Fallback to safe value. */
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+ bs->bl.request_alignment = (align != 1) ? align : max_align;
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break;
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}
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}
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qemu_vfree(buf);
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}
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- if (!bs->bl.request_alignment) {
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+ if (!s->buf_align) {
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+ int i;
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size_t align;
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- buf = qemu_memalign(s->buf_align, max_align);
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- for (align = 512; align <= max_align; align <<= 1) {
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- if (raw_is_io_aligned(fd, buf, align)) {
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- bs->bl.request_alignment = align;
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+ buf = qemu_memalign(max_align, 2 * max_align);
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+ for (i = 0; i < ARRAY_SIZE(alignments); i++) {
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+ align = alignments[i];
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+ if (raw_is_io_aligned(fd, buf + align, max_align)) {
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+ /* Fallback to request_aligment. */
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+ s->buf_align = (align != 1) ? align : bs->bl.request_alignment;
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break;
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}
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}
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--
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1.8.3.1
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