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Message-ID: <c384c5ea0908030823y494ee8bei85aa62c1470fa3ec@mail.gmail.com>
Date: Mon, 3 Aug 2009 17:23:44 +0200
From: Leon Woestenberg <leon.woestenberg@...il.com>
To: linux-kernel@...r.kernel.org
Subject: get_user_pages() on an mmap()ed file allowed? What to do if 0 <
get_user_pages(..., nr_pages, ...) < nr_pages?
Hello,
I have a PCI device driver performing DMA to a scattered user-space buffer.
Given a malloc()ed buffer, get_user_pages(..., buffer, nr_pages, ...)
always returns to requested number of pages and everything works as
expected.
So far so good.
Since that I changed userspace to mmap() a file, instead of
malloc()ing a buffer.
The mmap() in userspace works.
However, in the driver get_user_pages() starts to return less pages
than I requested, in an undeterministic fashion (most of the times I
get the expected number,
sometimes I get only a part of the requested pages).
Reading the get_user_pages() implementation dazzles me too much,
still. I wonder if I am violating the kernel API?
- is it allowed to have a PCI device DMA-read from memory pages, that
belong to a file mmap()'d by userspace?
- what are valid reasons for get_user_pages() to fail?
- what should a driver do when get_user_pages() returns less pages
than requested?
A snippet of the code:
my user space does:
int fd = open(filename, O_RDONLY);
assert(fd >= 0);
/* map the file in memory */
char *buffer = mmap(0, buffersize, PROT_READ, MAP_SHARED, fd, 0);
assert(buffer != MAP_FAILED);
/* advice sequential access */
int rc = madvise(buffer, buffersize, MADV_SEQUENTIAL);
assert(rc == 0);
my driver does:
const unsigned long first = (boe & PAGE_MASK) >> PAGE_SHIFT;
const unsigned long last = ((boe + count - 1) & PAGE_MASK) >> PAGE_SHIFT;
const int nr_pages = last - first + 1;
...
down_read(¤t->mm->mmap_sem);
rc = get_user_pages(current, current->mm, start & PAGE_MASK,
nr_pages, 0 /* do not write*/, 1 /* do force */, pages, NULL);
up_read(¤t->mm->mmap_sem);
BUG_ON(rc < nr_pages);
Thanks,
--
Leon
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