[PATCH] process_vm_readv.2: correct partial transfer granularity

From: Alban Crequy <[email protected]>

The man page claimed that partial transfers apply at the granularity
of iovec elements and that these system calls won't split a single
iovec element. This is correct for local iovecs (which are validated
upfront) but incorrect for remote iovecs: the kernel processes remote
memory at page granularity via pin_user_pages_remote(), so a partial
transfer can occur within a single remote iovec element at a page
boundary.

For example, if a remote iovec spans two pages and the second page is
unmapped, the syscall returns the number of bytes from the first page
(a partial transfer within one iovec element), not -1/EFAULT.

I verified this by testing process_vm_readv() and process_vm_writev()
with a 2-page remote iovec where the second page was unmapped via
munmap(). Both returned one page worth of bytes (a short read/write),
confirming page-granularity partial transfers.

I was prompted to check the current behaviour by the LKML review on
my patch (not merged yet) on process_vm_readv adding new flags:
https://lore.kernel.org/lkml/[email protected]/

The inaccurate text was introduced in commit 0b01869b0a59
("process_vm_readv.2: Cleanups after comments from Mike Frysinger
and Christopher Yeoh", 2012-04-14), based on a misinterpretation of
a review comment. The original man page text by Christopher Yeoh
correctly described partial transfers without claiming iovec-element
granularity.

Note: process_vm_writev.2 is a .so redirect to process_vm_readv.2,
so this fix applies to both pages.

Fixes: 0b01869b0a59 ("process_vm_readv.2: Cleanups after comments from Mike Frysinger and Christopher Yeoh")
Signed-off-by: Alban Crequy <[email protected]>
---
 man/man2/process_vm_readv.2 | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/man/man2/process_vm_readv.2 b/man/man2/process_vm_readv.2
index 66554a2cf..2ccfe60d0 100644
--- a/man/man2/process_vm_readv.2
+++ b/man/man2/process_vm_readv.2
@@ -192,10 +192,13 @@ returns the number of bytes read and
 returns the number of bytes written.
 This return value may be less than the total number of requested bytes,
 if a partial read/write occurred.
-(Partial transfers apply at the granularity of
+(Partial transfers apply at the granularity of pages in the remote process.
+If a remote
 .I iovec
-elements.
-These system calls won't perform a partial transfer that splits a single
+element spans multiple pages and one of those pages is invalid
+or not resident,
+data may be transferred from/to the pages preceding the failing page,
+resulting in a partial transfer that splits a single
 .I iovec
 element.)
 The caller should check the return value to determine whether
-- 
2.45.0