Re: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"

From: John Hubbard
Date: Mon Jun 01 2020 - 01:11:52 EST

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.

John Hubbard