On Sun, Jun 17, 2018 at 1:04 PM, Jason Gunthorpe <jgg@xxxxxxxx> wrote: > On Sun, Jun 17, 2018 at 12:53:04PM -0700, Dan Williams wrote: >> > diff --git a/mm/rmap.c b/mm/rmap.c >> > index 6db729dc4c50..37576f0a4645 100644 >> > +++ b/mm/rmap.c >> > @@ -1360,6 +1360,8 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, >> > flags & TTU_SPLIT_FREEZE, page); >> > } >> > >> > + if (PageDmaPinned(page)) >> > + return false; >> > /* >> > * We have to assume the worse case ie pmd for invalidation. Note that >> > * the page can not be free in this function as call of try_to_unmap() >> >> We have a similiar problem with DAX and the conclusion we came to is >> that it is not acceptable for userspace to arbitrarily block kernel >> actions. The conclusion there was: 'wait' if the DMA is transient, and >> 'revoke' if the DMA is long lived, or otherwise 'block' long-lived DMA >> if a revocation mechanism is not available. > > This might be the right answer for certain things, but it shouldn't be > the immediate reaction to everthing. There are many user APIs that > block kernel actions and hold kernel resources. > > IMHO, there should be an identifiable objection, eg is blocking going > to create a DOS, dead-lock, insecurity, etc? I believe kernel behavior regression is a primary concern as now fallocate() and truncate() can randomly fail where they didn't before.
