2e148f1e3d
Currently we have a zap_mapping pointer maintained in zap_details, when it is specified we only want to zap the pages that has the same mapping with what the caller has specified. But what we want to do is actually simpler: we want to skip zapping private (COW-ed) pages in some cases. We can refer to unmap_mapping_pages() callers where we could have passed in different even_cows values. The other user is unmap_mapping_folio() where we always want to skip private pages. According to Hugh, we used a mapping pointer for historical reason, as explained here: https://lore.kernel.org/lkml/391aa58d-ce84-9d4-d68d-d98a9c533255@google.com/ Quoting partly from Hugh: Which raises the question again of why I did not just use a boolean flag there originally: aah, I think I've found why. In those days there was a horrible "optimization", for better performance on some benchmark I guess, which when you read from /dev/zero into a private mapping, would map the zero page there (look up read_zero_pagealigned() and zeromap_page_range() if you dare). So there was another category of page to be skipped along with the anon COWs, and I didn't want multiple tests in the zap loop, so checking check_mapping against page->mapping did both. I think nowadays you could do it by checking for PageAnon page (or genuine swap entry) instead. This patch replaces the zap_details.zap_mapping pointer into the even_cows boolean, then we check it against PageAnon. Link: https://lkml.kernel.org/r/20220216094810.60572-4-peterx@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com> Suggested-by: Hugh Dickins <hughd@google.com> Reviewed-by: John Hubbard <jhubbard@nvidia.com> Cc: David Hildenbrand <david@redhat.com> Cc: Alistair Popple <apopple@nvidia.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: "Kirill A . Shutemov" <kirill@shutemov.name> Cc: Matthew Wilcox <willy@infradead.org> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Yang Shi <shy828301@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.