Messages in this thread | | | Subject | Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions | From | Tom Talpey <> | Date | Tue, 19 Mar 2019 15:55:23 -0500 |
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On 3/19/2019 3:45 PM, Jerome Glisse wrote: > On Tue, Mar 19, 2019 at 03:43:44PM -0500, Tom Talpey wrote: >> On 3/19/2019 4:03 AM, Ira Weiny wrote: >>> On Tue, Mar 19, 2019 at 04:36:44PM +0100, Jan Kara wrote: >>>> On Tue 19-03-19 17:29:18, Kirill A. Shutemov wrote: >>>>> On Tue, Mar 19, 2019 at 10:14:16AM -0400, Jerome Glisse wrote: >>>>>> On Tue, Mar 19, 2019 at 09:47:24AM -0400, Jerome Glisse wrote: >>>>>>> On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote: >>>>>>>> On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@gmail.com wrote: >>>>>>>>> From: John Hubbard <jhubbard@nvidia.com> >>>>>>> >>>>>>> [...] >>>>>>> >>>>>>>>> diff --git a/mm/gup.c b/mm/gup.c >>>>>>>>> index f84e22685aaa..37085b8163b1 100644 >>>>>>>>> --- a/mm/gup.c >>>>>>>>> +++ b/mm/gup.c >>>>>>>>> @@ -28,6 +28,88 @@ struct follow_page_context { >>>>>>>>> unsigned int page_mask; >>>>>>>>> }; >>>>>>>>> +typedef int (*set_dirty_func_t)(struct page *page); >>>>>>>>> + >>>>>>>>> +static void __put_user_pages_dirty(struct page **pages, >>>>>>>>> + unsigned long npages, >>>>>>>>> + set_dirty_func_t sdf) >>>>>>>>> +{ >>>>>>>>> + unsigned long index; >>>>>>>>> + >>>>>>>>> + for (index = 0; index < npages; index++) { >>>>>>>>> + struct page *page = compound_head(pages[index]); >>>>>>>>> + >>>>>>>>> + if (!PageDirty(page)) >>>>>>>>> + sdf(page); >>>>>>>> >>>>>>>> How is this safe? What prevents the page to be cleared under you? >>>>>>>> >>>>>>>> If it's safe to race clear_page_dirty*() it has to be stated explicitly >>>>>>>> with a reason why. It's not very clear to me as it is. >>>>>>> >>>>>>> The PageDirty() optimization above is fine to race with clear the >>>>>>> page flag as it means it is racing after a page_mkclean() and the >>>>>>> GUP user is done with the page so page is about to be write back >>>>>>> ie if (!PageDirty(page)) see the page as dirty and skip the sdf() >>>>>>> call while a split second after TestClearPageDirty() happens then >>>>>>> it means the racing clear is about to write back the page so all >>>>>>> is fine (the page was dirty and it is being clear for write back). >>>>>>> >>>>>>> If it does call the sdf() while racing with write back then we >>>>>>> just redirtied the page just like clear_page_dirty_for_io() would >>>>>>> do if page_mkclean() failed so nothing harmful will come of that >>>>>>> neither. Page stays dirty despite write back it just means that >>>>>>> the page might be write back twice in a row. >>>>>> >>>>>> Forgot to mention one thing, we had a discussion with Andrea and Jan >>>>>> about set_page_dirty() and Andrea had the good idea of maybe doing >>>>>> the set_page_dirty() at GUP time (when GUP with write) not when the >>>>>> GUP user calls put_page(). We can do that by setting the dirty bit >>>>>> in the pte for instance. They are few bonus of doing things that way: >>>>>> - amortize the cost of calling set_page_dirty() (ie one call for >>>>>> GUP and page_mkclean() >>>>>> - it is always safe to do so at GUP time (ie the pte has write >>>>>> permission and thus the page is in correct state) >>>>>> - safe from truncate race >>>>>> - no need to ever lock the page >>>>>> >>>>>> Extra bonus from my point of view, it simplify thing for my generic >>>>>> page protection patchset (KSM for file back page). >>>>>> >>>>>> So maybe we should explore that ? It would also be a lot less code. >>>>> >>>>> Yes, please. It sounds more sensible to me to dirty the page on get, not >>>>> on put. >>>> >>>> I fully agree this is a desirable final state of affairs. >>> >>> I'm glad to see this presented because it has crossed my mind more than once >>> that effectively a GUP pinned page should be considered "dirty" at all times >>> until the pin is removed. This is especially true in the RDMA case. >> >> But, what if the RDMA registration is readonly? That's not uncommon, and >> marking dirty unconditonally would add needless overhead to such pages. > > Yes and this is only when FOLL_WRITE is set ie when you are doing GUP and > asking for write. Doing GUP and asking for read is always safe.
Aha, ok great.
I guess it does introduce something for callers to be aware of, if they GUP very large regions. I suppose if they're sufficiently aware of the situation, e.g. pnfs LAYOUT_COMMIT notifications, they could walk lists and reset page_dirty for untouched pages before releasing. That's their issue though, and agreed it's safest for the GUP layer to mark.
Tom.
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