Messages in this thread | | | Subject | Re: [PATCH v1 1/1] fs/splice: add missing callback for inaccessible pages | From | John Hubbard <> | Date | Thu, 30 Apr 2020 15:26:25 -0700 |
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On 2020-04-30 12:54, Christian Borntraeger wrote: > On 30.04.20 21:02, Christian Borntraeger wrote: >> On 30.04.20 20:12, Christian Borntraeger wrote: >>> On 29.04.20 18:07, Dave Hansen wrote: >>>> On 4/28/20 3:50 PM, Claudio Imbrenda wrote: >>>>> If a page is inaccesible and it is used for things like sendfile, then >>>>> the content of the page is not always touched, and can be passed >>>>> directly to a driver, causing issues. >>>>> >>>>> This patch fixes the issue by adding a call to arch_make_page_accessible >>>>> in page_cache_pipe_buf_confirm; this fixes the issue. >>>> >>>> I spent about 5 minutes putting together a patch: >>>> >>>> https://sr71.net/~dave/intel/accessible.patch >>> >>> You only set the page flag for compound pages. that of course leaves a big pile >>> of pages marked a not accessible, thus explaining the sendto trace and all kind >>> of other random traces. >>> >>> >>> What do you see when you also do the SetPageAccessible(page); >>> in the else page of prep_new_page (order == 0). >>> (I do get > 10000 of these non compound page allocs just during boot). >>> >> >> And yes, I think you are right that we should call the callback also for !FOLL_PIN. >
Disclaimer: I haven't dug into the details of the latest points above, so answers below will be narrowly focused.
> > Thinking again about this I am no longer sure. Adding John Hubbard. > > Documentation/core-api/pin_user_pages.rst says: > -------snip---------- > Another way of thinking about these flags is as a progression of restrictions: > FOLL_GET is for struct page manipulation, without affecting the data that the > struct page refers to. FOLL_PIN is a *replacement* for FOLL_GET, and is for > short term pins on pages whose data *will* get accessed. As such, FOLL_PIN is > a "more severe" form of pinning. And finally, FOLL_LONGTERM is an even more > restrictive case that has FOLL_PIN as a prerequisite: this is for pages that > will be pinned longterm, and whose data will be accessed. > -------snip---------- > > So John,is it ok to give a page to an I/O device where the code has used gup > with FOLL_GET (or gup fast without pup) or would you consider this a bug? >
Well, it's a bug (or a bug-in-waiting): even though gup/FOLL_GET works just as well (and as badly) as ever, pup/FOLL_PIN is required in order to safely and correctly allow a non-CPU device to operate on a page's data. Core mm and fs code is going to key off of page_maybe_dma_pinned() in order to make critical decisions about writeback and umount, and FOLL_PIN opts into that; FOLL_GET does not.
Basically, you'd be creating another set of call sites that someone would have to convert to pup/FOLL_PIN.
btw, on the FOLL_LONGTERM documentation above: that's more of an aspiration than a description of current behavior, in some ways. The current FOLL_LONGTERM is a little more quirky than is implied there.
Also on a related note, I've been slow in posting patches to implement the remaining call site conversions, and am trying to get back to that asap. There have been some distractions. :) Once every call site is correctly using gup or pup, it will be easier for everyone.
thanks, -- John Hubbard NVIDIA
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