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File-backed memory differs from anonymous memory in that even if the pte is missing, the data could still resides either in the file or in page/swap cache. So when wr-protect a pte, we need to consider none ptes too. We do that by installing the uffd-wp pte markers when necessary. So when there's a future write to the pte, the fault handler will go the special path to first fault-in the page as read-only, then report to userfaultfd server with the wr-protect message. On the other hand, when unprotecting a page, it's also possible that the pte got unmapped but replaced by the special uffd-wp marker. Then we'll need to be able to recover from a uffd-wp pte marker into a none pte, so that the next access to the page will fault in correctly as usual when accessed the next time. Special care needs to be taken throughout the change_protection_range() process. Since now we allow user to wr-protect a none pte, we need to be able to pre-populate the page table entries if we see (!anonymous && MM_CP_UFFD_WP) requests, otherwise change_protection_range() will always skip when the pgtable entry does not exist. For example, the pgtable can be missing for a whole chunk of 2M pmd, but the page cache can exist for the 2M range. When we want to wr-protect one 4K page within the 2M pmd range, we need to pre-populate the pgtable and install the pte marker showing that we want to get a message and block the thread when the page cache of that 4K page is written. Without pre-populating the pmd, change_protection() will simply skip that whole pmd. Note that this patch only covers the small pages (pte level) but not covering any of the transparent huge pages yet. That will be done later, and this patch will be a preparation for it too. Link: https://lkml.kernel.org/r/20220405014850.14352-1-peterx@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com> Cc: Alistair Popple <apopple@nvidia.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Axel Rasmussen <axelrasmussen@google.com> Cc: David Hildenbrand <david@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: Jerome Glisse <jglisse@redhat.com> Cc: "Kirill A . Shutemov" <kirill@shutemov.name> Cc: Matthew Wilcox <willy@infradead.org> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com> Cc: Nadav Amit <nadav.amit@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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| README | ||
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.