On 22.03.22 17:39, Andrew Deason wrote: > On Wed, 16 Mar 2022 10:41:41 +0100 > David Hildenbrand <da...@redhat.com> wrote: > >> On 16.03.22 10:37, Dr. David Alan Gilbert wrote: >>> * Peter Maydell (peter.mayd...@linaro.org) wrote: >>>> On Wed, 16 Mar 2022 at 07:53, David Hildenbrand <da...@redhat.com> wrote: >>>>> >>>>> On 16.03.22 05:04, Andrew Deason wrote: >>>>>> We have a thin wrapper around madvise, called qemu_madvise, which >>>>>> provides consistent behavior for the !CONFIG_MADVISE case, and works >>>>>> around some platform-specific quirks (some platforms only provide >>>>>> posix_madvise, and some don't offer all 'advise' types). This specific >>>>>> caller of madvise has never used it, tracing back to its original >>>>>> introduction in commit e0b266f01dd2 ("migration_completion: Take >>>>>> current state"). >>>>>> >>>>>> Call qemu_madvise here, to follow the same logic as all of our other >>>>>> madvise callers. This slightly changes the behavior for >>>>>> !CONFIG_MADVISE (EINVAL instead of ENOSYS, and a slightly different >>>>>> error message), but this is now more consistent with other callers >>>>>> that use qemu_madvise. >>>>>> >>>>>> Signed-off-by: Andrew Deason <adea...@sinenomine.net> >>>>>> --- >>>>>> Looking at the history of commits that touch this madvise() call, it >>>>>> doesn't _look_ like there's any reason to be directly calling madvise vs >>>>>> qemu_advise (I don't see anything mentioned), but I'm not sure. >>>>>> >>>>>> softmmu/physmem.c | 12 ++---------- >>>>>> 1 file changed, 2 insertions(+), 10 deletions(-) >>>>>> >>>>>> diff --git a/softmmu/physmem.c b/softmmu/physmem.c >>>>>> index 43ae70fbe2..900c692b5e 100644 >>>>>> --- a/softmmu/physmem.c >>>>>> +++ b/softmmu/physmem.c >>>>>> @@ -3584,40 +3584,32 @@ int ram_block_discard_range(RAMBlock *rb, >>>>>> uint64_t start, size_t length) >>>>>> rb->idstr, start, length, ret); >>>>>> goto err; >>>>>> #endif >>>>>> } >>>>>> if (need_madvise) { >>>>>> /* 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) >>>>>> if (qemu_ram_is_shared(rb) && rb->fd < 0) { >>>>>> - ret = madvise(host_startaddr, length, QEMU_MADV_REMOVE); >>>>>> + ret = qemu_madvise(host_startaddr, length, >>>>>> QEMU_MADV_REMOVE); >>>>>> } else { >>>>>> - ret = madvise(host_startaddr, length, >>>>>> QEMU_MADV_DONTNEED); >>>>>> + ret = qemu_madvise(host_startaddr, length, >>>>>> QEMU_MADV_DONTNEED); >>>>> >>>>> posix_madvise(QEMU_MADV_DONTNEED) has completely different semantics >>>>> then madvise() -- it's not a discard that we need here. >>>>> >>>>> So ram_block_discard_range() would now succeed in environments (BSD?) >>>>> where it's supposed to fail. >>>>> >>>>> So AFAIKs this isn't sane. >>>> >>>> But CONFIG_MADVISE just means "host has madvise()"; it doesn't imply >>>> "this is a Linux madvise() with MADV_DONTNEED". Solaris madvise() >>>> doesn't seem to have MADV_DONTNEED at all; a quick look at the >>>> FreeBSD manpage suggests its madvise MADV_DONTNEED is identical >>>> to its posix_madvise MADV_DONTNEED. >>>> >>>> If we need "specifically Linux MADV_DONTNEED semantics" maybe we >>>> should define a QEMU_MADV_LINUX_DONTNEED which either (a) does the >>>> right thing or (b) fails, and use qemu_madvise() regardless. >>>> >>>> Certainly the current code is pretty fragile to being changed by >>>> people who don't understand the undocumented subtlety behind >>>> the use of a direct madvise() call here. >>> >>> Yeh and I'm not sure I can remembe rall the subtleties; there's a big >>> hairy set of ifdef's in include/qemu/madvise.h that makes >>> sure we always have the definition of QEMU_MADV_REMOVE/DONTNEED >>> even on platforms that might not define it themselves. >>> >>> But I think this code is used for things with different degrees >>> of care about the semantics; e.g. 'balloon' just cares that >>> it frees memory up and doesn't care about the detailed semantics >>> that much; so it's probably fine with that. >>> Postcopy is much more touchy, but then it's only going to be >>> calling this on Linux anyway (because of the userfault dependency). >> >> MADV_DONTNEED/MADV_REMOVE only provides discard semantics on Linux IIRC >> -- and that's what we want to achieve: ram_block_discard_range() >> >> So I agree with Peter that we might want to make this more explicit. > > I was looking at the comments/history around this code to try to make > this more explicit/clear, and it seems like the whole function is very > Linux-specific. All we ever do is: > > - fallocate(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE) > - madvise(MADV_REMOVE) > - madvise(MADV_DONTNEED) with Linux semantics > > All of those operations are Linux-only, so trying to figure out the > cross-platform way to model this seems kind of pointless. Is it fine to > just #ifdef the whole thing to be just for Linux? >
Fine with me, as long as it compiles on other OSs :) -- Thanks, David / dhildenb