> On Jun 22, 2021, at 11:36 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Tue, Jun 22, 2021 at 11:28:30AM -0700, Linus Torvalds wrote: >> On Tue, Jun 22, 2021 at 11:23 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: >>> >>> It wouldn't be _that_ bad necessarily. filemap_fault: >> >> It's not actually the mm code that is the biggest problem. We >> obviously already have readahead support. >> >> It's the *fault* side. >> >> In particular, since the fault would return without actually filling >> in the page table entry (because the page isn't ready yet, and you >> cannot expose it to other threads!), you also have to jump over the >> instruction that caused this all. > > Oh, I was assuming that it'd be a function call like > get_user_pages_fast(), not an instruction that was specially marked to > be jumped over. Gag reflex diminishing now? Just reminding the alternative (in the RFC that I mentioned before): a vDSO exception table entry for a memory accessing function in the vDSO. It then behaves as a sort of MADV_WILLNEED for the faulting page if an exception is triggered. Unlike MADV_WILLNEED it maps the page if no IO is needed. It can return through a register whether the page was present or not. I once implemented (another) alternative, in which the ELF had a section with an exception-table (holding all the “Async-#PF” instructions), which described where to skip to if a #PF occurs, but this solution seemed too heavy-weight/intrusive.
