Re: [PATCH v1] madvise.2: Document MADV_POPULATE_READ and MADV_POPULATE_WRITE

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On 12.07.21 11:58, Pankaj Gupta wrote:
MADV_POPULATE_READ and MADV_POPULATE_WRITE have been merged into
upstream Linux via commit 4ca9b3859dac ("mm/madvise: introduce
MADV_POPULATE_(READ|WRITE) to prefault page tables"), part of v5.14-rc1.

Let's document the behavior and error conditions of these new madvise()
options.

Cc: Alejandro Colomar <alx.manpages@xxxxxxxxx>
Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Linux API <linux-api@xxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
  man2/madvise.2 | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 80 insertions(+)

diff --git a/man2/madvise.2 b/man2/madvise.2
index f1f384c0c..3ec8c53a7 100644
--- a/man2/madvise.2
+++ b/man2/madvise.2
@@ -469,6 +469,59 @@ If a page is file-backed and dirty, it will be written back to the backing
  storage.
  The advice might be ignored for some pages in the range when it is not
  applicable.
+.TP
+.BR MADV_POPULATE_READ " (since Linux 5.14)
+Populate (prefault) page tables readable for the whole range without actually
+reading. Depending on the underlying mapping, map the shared zeropage,
+preallocate memory or read the underlying file; files with holes might or
+might not preallocate blocks.
+Do not generate
+.B SIGBUS
+when populating fails, return an error instead.
+.IP
+If
+.B MADV_POPULATE_READ
+succeeds, all page tables have been populated (prefaulted) readable once.
+If
+.B MADV_POPULATE_READ
+fails, some page tables might have been populated.
+.IP
+.B MADV_POPULATE_READ
+cannot be applied to mappings without read permissions
+and special mappings marked with the kernel-internal
+.B VM_PFNMAP
+and
+.BR VM_IO .
+.IP
+Note that with
+.BR MADV_POPULATE_READ ,
+the process can be killed at any moment when the system runs out of memory.
+.TP
+.BR MADV_POPULATE_WRITE " (since Linux 5.14)
+Populate (prefault) page tables writable for the whole range without actually
+writing. Depending on the underlying mapping, preallocate memory or read the

Is this read or write?
just reading and trying to understand :)

It's reading. Assume you have a file with existing content mapped into a process. Once you touch a page (read/write/execute) that maps to blocks with existing content, you'll have to load these blocks from disk first.

Thanks! :)

--
Thanks,

David / dhildenb




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