Messages in this thread | | | From | John Hubbard <> | Subject | Re: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5" | Date | Sun, 31 May 2020 22:11:49 -0700 |
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On 2020-05-31 00:11, Souptick Joarder wrote: ... >> diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst >> index 4675b04e8829..b9f2688a2c67 100644 >> --- a/Documentation/core-api/pin_user_pages.rst >> +++ b/Documentation/core-api/pin_user_pages.rst >> @@ -171,6 +171,26 @@ If only struct page data (as opposed to the actual memory contents that a page >> is tracking) is affected, then normal GUP calls are sufficient, and neither flag >> needs to be set. >> >> +CASE 5: Pinning in order to write to the data within the page >> +------------------------------------------------------------- >> +Even though neither DMA nor Direct IO is involved, just a simple case of "pin, >> +access page's data, unpin" can cause a problem. > > Will it be, *"pin, access page's data, set page dirty, unpin" * ?
Well...the problem can show up with just accessing (writing) the data. But it is true that this statement is a little different from the patterns below, which is confusing. I'll delete set_page_dirty() from each of them, in order to avoid confusing things. (Although each is correct.) And I'll also change the above to "pin, write to a page's data, upin".
set_page_dirty() interactions are really just extra credit here. :) And fully read-only situations won't cause a problem.
> > Case 5 may be considered a >> +superset of Case 1, plus Case 2, plus anything that invokes that pattern. In >> +other words, if the code is neither Case 1 nor Case 2, it may still require >> +FOLL_PIN, for patterns like this: >> + >> +Correct (uses FOLL_PIN calls): >> + pin_user_pages() >> + access the data within the pages >> + set_page_dirty_lock() >> + unpin_user_pages() >> + >> +INCORRECT (uses FOLL_GET calls): >> + get_user_pages() >> + access the data within the pages >> + set_page_dirty_lock() >> + put_page() >> +
I'll send a v2 shortly.
thanks, -- John Hubbard NVIDIA
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