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List:       linux-api
Subject:    Re: [PATCH v3] madvise.2: Document MADV_POPULATE_READ and MADV_POPULATE_WRITE
From:       David Hildenbrand <david () redhat ! com>
Date:       2021-09-28 16:34:31
Message-ID: 50357269-d227-5fda-a450-c47b035b9586 () redhat ! com
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On 23.08.21 14:06, David Hildenbrand 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.
> 
> Further, commit eb2faa513c24 ("mm/madvise: report SIGBUS as -EFAULT for
> MADV_POPULATE_(READ|WRITE)"), part of v5.14-rc6, made sure that SIGBUS is
> converted to -EFAULT instead of -EINVAL.
> 
> Let's document the behavior and error conditions of these new madvise()
> options.
> 
> Acked-by: Pankaj Gupta <pankaj.gupta@ionos.com>
> Cc: Alejandro Colomar <alx.manpages@gmail.com>
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Jann Horn <jannh@google.com>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Linux API <linux-api@vger.kernel.org>
> Cc: linux-mm@kvack.org
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> 
> v2 -> v3:
> - Refine what "populating readable/writable" means
> - Compare each version with MAP_POPULATE and give an example use case
> - Reword SIGBUS handling
> - Reword comment regarding special mappings and also add memfd_secret(2)
> - Reference MADV_HWPOISON when talking about HW poisoned pages
> - Minor cosmetic fixes
> 
> v1 -> v2:
> - Use semantic newlines in all cases
> - Add two missing "
> - Document -EFAULT handling
> - Rephrase some parts to make it more generic: VM_PFNMAP and VM_IO are only
>    examples for special mappings
> 
> ---
>   man2/madvise.2 | 156 +++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 156 insertions(+)
> 
> diff --git a/man2/madvise.2 b/man2/madvise.2
> index f1f384c0c..37f6dd6fa 100644
> --- a/man2/madvise.2
> +++ b/man2/madvise.2
> @@ -469,6 +469,106 @@ 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,
> +faulting in all pages in the range just as if manually reading from each page;
> +however,
> +avoid the actual memory access that would have been performed after handling
> +the fault.
> +.IP
> +In contrast to
> +.BR MAP_POPULATE ,
> +.B MADV_POPULATE_READ
> +does not hide errors,
> +can be applied to (parts of) existing mappings and will always populate
> +(prefault) page tables readable.
> +One example use case is prefaulting a file mapping,
> +reading all file content from disk;
> +however,
> +pages won't be dirtied and consequently won't have to be written back to disk
> +when evicting the pages from memory.
> +.IP
> +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.
> +If populating fails,
> +a
> +.B SIGBUS
> +signal is not generated; instead, an error is returned.
> +.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,
> +for example,
> +mappings marked with kernel-internal flags such as
> +.B VM_PFNMAP
> +or
> +.BR VM_IO ,
> +or secret memory regions created using
> +.BR memfd_secret(2) .
> +.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,
> +faulting in all pages in the range just as if manually writing to each
> +each page;
> +however,
> +avoid the actual memory access that would have been performed after handling
> +the fault.
> +.IP
> +In contrast to
> +.BR MAP_POPULATE ,
> +MADV_POPULATE_WRITE does not hide errors,
> +can be applied to (parts of) existing mappings and will always populate
> +(prefault) page tables writable.
> +One example use case is preallocating memory,
> +breaking any CoW (Copy on Write).
> +.IP
> +Depending on the underlying mapping,
> +preallocate memory or read the underlying file;
> +files with holes will preallocate blocks.
> +If populating fails,
> +a
> +.B SIGBUS
> +signal is not generated; instead, an error is returned.
> +.IP
> +If
> +.B MADV_POPULATE_WRITE
> +succeeds,
> +all page tables have been populated (prefaulted) writable once.
> +If
> +.B MADV_POPULATE_WRITE
> +fails,
> +some page tables might have been populated.
> +.IP
> +.B MADV_POPULATE_WRITE
> +cannot be applied to mappings without write permissions
> +and special mappings,
> +for example,
> +mappings marked with kernel-internal flags such as
> +.B VM_PFNMAP
> +or
> +.BR VM_IO ,
> +or secret memory regions created using
> +.BR memfd_secret(2) .
> +.IP
> +Note that with
> +.BR MADV_POPULATE_WRITE ,
> +the process can be killed at any moment when the system runs out of memory.
>   .SH RETURN VALUE
>   On success,
>   .BR madvise ()
> @@ -490,6 +590,22 @@ A kernel resource was temporarily unavailable.
>   .B EBADF
>   The map exists, but the area maps something that isn't a file.
>   .TP
> +.B EFAULT
> +.I advice
> +is
> +.B MADV_POPULATE_READ
> +or
> +.BR MADV_POPULATE_WRITE ,
> +and populating (prefaulting) page tables failed because a
> +.B SIGBUS
> +would have been generated on actual memory access and the reason is not a
> +HW poisoned page
> +(HW poisoned pages can,
> +for example,
> +be created using the
> +.B MADV_HWPOISON
> +flag described elsewhere in this page).
> +.TP
>   .B EINVAL
>   .I addr
>   is not page-aligned or
> @@ -533,6 +649,22 @@ or
>   .BR VM_PFNMAP
>   ranges.
>   .TP
> +.B EINVAL
> +.I advice
> +is
> +.B MADV_POPULATE_READ
> +or
> +.BR MADV_POPULATE_WRITE ,
> +but the specified address range includes ranges with insufficient permissions
> +or special mappings,
> +for example,
> +mappings marked with kernel-internal flags such a
> +.B VM_IO
> +or
> +.BR VM_PFNMAP ,
> +or secret memory regions created using
> +.BR memfd_secret(2) .
> +.TP
>   .B EIO
>   (for
>   .BR MADV_WILLNEED )
> @@ -548,6 +680,15 @@ Not enough memory: paging in failed.
>   Addresses in the specified range are not currently
>   mapped, or are outside the address space of the process.
>   .TP
> +.B ENOMEM
> +.I advice
> +is
> +.B MADV_POPULATE_READ
> +or
> +.BR MADV_POPULATE_WRITE ,
> +and populating (prefaulting) page tables failed because there was not enough
> +memory.
> +.TP
>   .B EPERM
>   .I advice
>   is
> @@ -555,6 +696,20 @@ is
>   but the caller does not have the
>   .B CAP_SYS_ADMIN
>   capability.
> +.TP
> +.B EHWPOISON
> +.I advice
> +is
> +.B MADV_POPULATE_READ
> +or
> +.BR MADV_POPULATE_WRITE ,
> +and populating (prefaulting) page tables failed because a HW poisoned page
> +(HW poisoned pages can,
> +for example,
> +be created using the
> +.B MADV_HWPOISON
> +flag described elsewhere in this page)
> +was encountered.
>   .SH VERSIONS
>   Since Linux 3.18,
>   .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
> @@ -602,6 +757,7 @@ from the system call, as it should).
>   .\" function first appeared in 4.4BSD.
>   .SH SEE ALSO
>   .BR getrlimit (2),
> +.BR memfd_secret(2),
>   .BR mincore (2),
>   .BR mmap (2),
>   .BR mprotect (2),
> 

Gentle ping.

-- 
Thanks,

David / dhildenb

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