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Re: [Qemu-devel] [PATCH v3 03/29] postcopy: use UFFDIO_ZEROPAGE only whe
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH v3 03/29] postcopy: use UFFDIO_ZEROPAGE only when available |
Date: |
Mon, 5 Mar 2018 17:23:24 +0000 |
User-agent: |
Mutt/1.9.2 (2017-12-15) |
* Peter Xu (address@hidden) wrote:
> On Fri, Feb 16, 2018 at 01:15:59PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> >
> > Use a flag on the RAMBlock to state whether it has the
> > UFFDIO_ZEROPAGE capability, use it when it's available.
> >
> > This allows the use of postcopy on tmpfs as well as hugepage
> > backed files.
> >
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> > exec.c | 15 +++++++++++++++
> > include/exec/cpu-common.h | 3 +++
> > migration/postcopy-ram.c | 13 ++++++++++---
> > 3 files changed, 28 insertions(+), 3 deletions(-)
> >
> > diff --git a/exec.c b/exec.c
> > index 0ec73bc917..1dc15298c2 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -99,6 +99,11 @@ static MemoryRegion io_mem_unassigned;
> > */
> > #define RAM_RESIZEABLE (1 << 2)
> >
> > +/* UFFDIO_ZEROPAGE is available on this RAMBlock to atomically
> > + * zero the page and wake waiting processes.
> > + * (Set during postcopy)
> > + */
> > +#define RAM_UF_ZEROPAGE (1 << 3)
> > #endif
> >
> > #ifdef TARGET_PAGE_BITS_VARY
> > @@ -1767,6 +1772,16 @@ bool qemu_ram_is_shared(RAMBlock *rb)
> > return rb->flags & RAM_SHARED;
> > }
> >
> > +bool qemu_ram_is_uf_zeroable(RAMBlock *rb)
> > +{
> > + return rb->flags & RAM_UF_ZEROPAGE;
> > +}
> > +
> > +void qemu_ram_set_uf_zeroable(RAMBlock *rb)
> > +{
> > + rb->flags |= RAM_UF_ZEROPAGE;
> > +}
> > +
> > /* Called with iothread lock held. */
> > void qemu_ram_set_idstr(RAMBlock *new_block, const char *name, DeviceState
> > *dev)
> > {
> > diff --git a/include/exec/cpu-common.h b/include/exec/cpu-common.h
> > index 0d861a6289..24d335f95d 100644
> > --- a/include/exec/cpu-common.h
> > +++ b/include/exec/cpu-common.h
> > @@ -73,6 +73,9 @@ void qemu_ram_set_idstr(RAMBlock *block, const char
> > *name, DeviceState *dev);
> > void qemu_ram_unset_idstr(RAMBlock *block);
> > const char *qemu_ram_get_idstr(RAMBlock *rb);
> > bool qemu_ram_is_shared(RAMBlock *rb);
> > +bool qemu_ram_is_uf_zeroable(RAMBlock *rb);
> > +void qemu_ram_set_uf_zeroable(RAMBlock *rb);
> > +
> > size_t qemu_ram_pagesize(RAMBlock *block);
> > size_t qemu_ram_pagesize_largest(void);
> >
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index bec6c2c66b..6297979700 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -490,6 +490,10 @@ static int ram_block_enable_notify(const char
> > *block_name, void *host_addr,
> > error_report("%s userfault: Region doesn't support COPY",
> > __func__);
> > return -1;
> > }
> > + if (reg_struct.ioctls & ((__u64)1 << _UFFDIO_ZEROPAGE)) {
> > + RAMBlock *rb = qemu_ram_block_by_name(block_name);
> > + qemu_ram_set_uf_zeroable(rb);
> > + }
>
> So the zeroable flag is only set after a listening operation of
> postcopy migration. One thing I am a bit worried is that if someone
> else wants to use the flag for a RAMBlock he/she may not notice this.
> Say, qemu_ram_is_uf_zeroable() is not valid if there is no such an
> incoming postcopy migration.
Yes, the problem is you don't get to know until you register it with
userfault.
> Maybe worth add a comment in the flag definition about this?
Yes, I've added:
/* Note: Only set at the start of postcopy */
above the qemu_ram_is_uf_zeroable() defintiion; also see the existing
'(Set during postcopy)' comment above the #define.
> Not a big deal (considering that I see no potential QEMU user for
> userfaultfd in short peroid), so no matter what:
>
> Reviewed-by: Peter Xu <address@hidden>
Thanks.
Dave
>
> >
> > return 0;
> > }
> > @@ -699,11 +703,14 @@ int postcopy_place_page(MigrationIncomingState *mis,
> > void *host, void *from,
> > int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
> > RAMBlock *rb)
> > {
> > + size_t pagesize = qemu_ram_pagesize(rb);
> > trace_postcopy_place_page_zero(host);
> >
> > - if (qemu_ram_pagesize(rb) == getpagesize()) {
> > - if (qemu_ufd_copy_ioctl(mis->userfault_fd, host, NULL,
> > getpagesize(),
> > - rb)) {
> > + /* Normal RAMBlocks can zero a page using UFFDIO_ZEROPAGE
> > + * but it's not available for everything (e.g. hugetlbpages)
> > + */
> > + if (qemu_ram_is_uf_zeroable(rb)) {
> > + if (qemu_ufd_copy_ioctl(mis->userfault_fd, host, NULL, pagesize,
> > rb)) {
> > int e = errno;
> > error_report("%s: %s zero host: %p",
> > __func__, strerror(e), host);
> > --
> > 2.14.3
> >
>
> --
> Peter Xu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
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