Messages in this thread | | | Date | Tue, 22 Jun 2021 19:23:10 +0100 | From | Matthew Wilcox <> | Subject | Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? |
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On Tue, Jun 22, 2021 at 11:07:56AM -0700, Linus Torvalds wrote: > On Tue, Jun 22, 2021 at 11:05 AM Matthew Wilcox <willy@infradead.org> wrote: > > > > Huh? Last I checked, the fault_in_readable actually read a byte from > > the page. It has to wait for the read to complete before that can > > happen. > > Yeah, we don't have any kind of async fault-in model. > > I'm not sure how that would even look. I don't think it would > necessarily be *impossible* (special marker in the exception table to > let the fault code know that this is a "prepare" fault), but it would > be pretty challenging.
It wouldn't be _that_ bad necessarily. filemap_fault:
page = find_get_page(mapping, offset); ... } else if (!page) { fpin = do_sync_mmap_readahead(vmf);
... and we could return at that point if the flag was set. There'd be some more details to fill in (if there's a !uptodate page in the page cache, don't wait for it), but it might not be too bad.
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