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    SubjectRe: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL
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    On Mon, 25 Oct 2021, Uladzislau Rezki wrote:
    > On Fri, Oct 22, 2021 at 09:49:08AM +1100, NeilBrown wrote:
    > > However I'm not 100% certain, and the behaviour might change in the
    > > future. So having one place (the definition of memalloc_retry_wait())
    > > where we can change the sleeping behaviour if the alloc_page behavour
    > > changes, would be ideal. Maybe memalloc_retry_wait() could take a
    > > gfpflags arg.
    > >
    > At sleeping is required for __get_vm_area_node() because in case of lack
    > of vmap space it will end up in tight loop without sleeping what is
    > really bad.
    >
    So vmalloc() has two failure modes. alloc_page() failure and
    __alloc_vmap_area() failure. The caller cannot tell which...

    Actually, they can. If we pass __GFP_NOFAIL to vmalloc(), and it fails,
    then it must have been __alloc_vmap_area() which failed.
    What do we do in that case?
    Can we add a waitq which gets a wakeup when __purge_vmap_area_lazy()
    finishes?
    If we use the spinlock from that waitq in place of free_vmap_area_lock,
    then the wakeup would be nearly free if no-one was waiting, and worth
    while if someone was waiting.

    Thanks,
    NeilBrown

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