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Re: [Qemu-devel] [RFC v2 03/32] migrate: Update ram_block_discard_range
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [RFC v2 03/32] migrate: Update ram_block_discard_range for shared |
Date: |
Mon, 18 Sep 2017 13:18:10 +0100 |
User-agent: |
Mutt/1.8.3 (2017-05-23) |
* Peter Xu (address@hidden) wrote:
> On Thu, Aug 24, 2017 at 08:27:01PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> >
> > The choice of call to discard a block is getting more complicated
> > for other cases. We use fallocate PUNCH_HOLE in any file cases;
> > it works for both hugepage and for tmpfs.
> > We use the DONTNEED for non-hugepage cases either where they're
> > anonymous or where they're private.
> >
> > Care should be taken when trying other backing files.
> >
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> > exec.c | 35 ++++++++++++++++++++++++-----------
> > trace-events | 3 +++
> > 2 files changed, 27 insertions(+), 11 deletions(-)
> >
> > diff --git a/exec.c b/exec.c
> > index d20c34ca83..67df2909ce 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -3573,6 +3573,7 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t
> > start, size_t length)
> > }
> >
> > if ((start + length) <= rb->used_length) {
> > + bool need_madvise, need_fallocate;
> > uint8_t *host_endaddr = host_startaddr + length;
> > if ((uintptr_t)host_endaddr & (rb->page_size - 1)) {
> > error_report("ram_block_discard_range: Unaligned end address:
> > %p",
> > @@ -3582,23 +3583,35 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t
> > start, size_t length)
> >
> > errno = ENOTSUP; /* If we are missing MADVISE etc */
> >
> > - if (rb->page_size == qemu_host_page_size) {
> > -#if defined(CONFIG_MADVISE)
> > - /* Note: We need the madvise MADV_DONTNEED behaviour of
> > definitely
> > - * freeing the page.
> > - */
> > - ret = madvise(host_startaddr, length, MADV_DONTNEED);
> > -#endif
> > - } else {
> > - /* Huge page case - unfortunately it can't do DONTNEED, but
> > - * it can do the equivalent by FALLOC_FL_PUNCH_HOLE in the
> > - * huge page file.
> > + /* The logic here is messy;
> > + * madvise DONTNEED fails for hugepages
> > + * fallocate works on hugepages and shmem
> > + */
> > + need_madvise = (rb->page_size == qemu_host_page_size);
> > + need_fallocate = rb->fd != -1;
> > + if (need_fallocate) {
> > + /* For a file, this causes the area of the file to be zero'd
> > + * if read, and for hugetlbfs also causes it to be unmapped
> > + * so a userfault will trigger.
> > */
> > #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
> > ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE |
> > FALLOC_FL_KEEP_SIZE,
> > start, length);
> > #endif
> > }
> > + /* i.e. need madvise but skip it if the fallocate failed */
> > + if (need_madvise && (!need_fallocate || (ret == 0))) {
>
> I'll slightly prefer:
>
> trace_ram_block_discard_range();
>
> if (need_fallocate) {
> ret = fallocate();
> if (ret) {
> error_report();
> goto err;
> }
> }
>
> if (need_madvise) {
> ret = madvise();
> if (ret) {
> error_report();
> goto err;
> }
> }
OK, I've reworked it more like that.
(It's a little more complex because of the ifdef's)
Dave
> But it is personal preference. For either way:
>
> Reviewed-by: Peter Xu <address@hidden>
>
> > + /* For normal RAM this causes it to be unmapped,
> > + * for shared memory it causes the local mapping to disappear
> > + * and to fall back on the file contents (which we just
> > + * fallocate'd away).
> > + */
> > +#if defined(CONFIG_MADVISE)
> > + ret = madvise(host_startaddr, length, MADV_DONTNEED);
> > +#endif
> > + }
> > + trace_ram_block_discard_range(rb->idstr, host_startaddr,
> > + need_madvise, need_fallocate, ret);
> > if (ret) {
> > ret = -errno;
> > error_report("ram_block_discard_range: Failed to discard range
> > "
> > diff --git a/trace-events b/trace-events
> > index 1f50f56d9d..213ee34f89 100644
> > --- a/trace-events
> > +++ b/trace-events
> > @@ -55,6 +55,9 @@ dma_complete(void *dbs, int ret, void *cb) "dbs=%p ret=%d
> > cb=%p"
> > dma_blk_cb(void *dbs, int ret) "dbs=%p ret=%d"
> > dma_map_wait(void *dbs) "dbs=%p"
> >
> > +# exec.c
> > +ram_block_discard_range(const char *rbname, void *hva, bool need_madvise,
> > bool need_fallocate, int ret) "address@hidden: madvise: %d fallocate: %d
> > ret: %d"
> > +
> > # memory.c
> > memory_region_ops_read(int cpu_index, void *mr, uint64_t addr, uint64_t
> > value, unsigned size) "cpu %d mr %p addr 0x%"PRIx64" value 0x%"PRIx64" size
> > %u"
> > memory_region_ops_write(int cpu_index, void *mr, uint64_t addr, uint64_t
> > value, unsigned size) "cpu %d mr %p addr 0x%"PRIx64" value 0x%"PRIx64" size
> > %u"
> > --
> > 2.13.5
> >
>
> --
> Peter Xu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
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