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Date: Tue, 19 Feb 2013 18:25:31 +0800
From: Simon Jeons <simon.jeons@...il.com>
To: Johannes Weiner <hannes@...xchg.org>
CC: Andrew Morton <akpm@...ux-foundation.org>,
Rusty Russell <rusty@...tcorp.com.au>,
LKML <linux-kernel@...r.kernel.org>,
Nick Piggin <npiggin@...e.de>,
Stewart Smith <stewart@...mingspork.com>, linux-mm@...ck.org,
linux-arch@...r.kernel.org
Subject: Re: [patch 1/2] mm: fincore()
Hi Johannes,
On 02/15/2013 02:34 PM, Johannes Weiner wrote:
> On Mon, Feb 11, 2013 at 02:12:39PM -0800, Andrew Morton wrote:
>> Also, having to mmap the file to be able to query pagecache state is a
>> hack. Whatever happened to the fincore() patch?
> I don't know, but how about this one:
>
> ---
> From: Johannes Weiner <hannes@...xchg.org>
> Subject: [patch 1/2] mm: fincore()
>
> Provide a syscall to determine whether a given file's pages are cached
> in memory. This is more elegant than mmapping the file for the sole
> purpose of using mincore(), and also works on NOMMU.
Who is the user of mincore()/fincore()? In which scenario user processes
need to know their pages are resident in memory or not?
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> ---
> include/linux/syscalls.h | 2 +
> mm/Makefile | 2 +-
> mm/fincore.c | 128 +++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 131 insertions(+), 1 deletion(-)
> create mode 100644 mm/fincore.c
>
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 313a8e0..3ceab2a 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -897,4 +897,6 @@ asmlinkage long sys_process_vm_writev(pid_t pid,
> asmlinkage long sys_kcmp(pid_t pid1, pid_t pid2, int type,
> unsigned long idx1, unsigned long idx2);
> asmlinkage long sys_finit_module(int fd, const char __user *uargs, int flags);
> +asmlinkage long sys_fincore(unsigned int fd, loff_t start, loff_t len,
> + unsigned char __user * vec);
> #endif
> diff --git a/mm/Makefile b/mm/Makefile
> index 185a22b..221cdae 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -17,7 +17,7 @@ obj-y := filemap.o mempool.o oom_kill.o fadvise.o \
> util.o mmzone.o vmstat.o backing-dev.o \
> mm_init.o mmu_context.o percpu.o slab_common.o \
> compaction.o balloon_compaction.o \
> - interval_tree.o $(mmu-y)
> + interval_tree.o fincore.o $(mmu-y)
>
> obj-y += init-mm.o
>
> diff --git a/mm/fincore.c b/mm/fincore.c
> new file mode 100644
> index 0000000..d504611
> --- /dev/null
> +++ b/mm/fincore.c
> @@ -0,0 +1,128 @@
> +#include <linux/syscalls.h>
> +#include <linux/pagemap.h>
> +#include <linux/file.h>
> +#include <linux/fs.h>
> +#include <linux/mm.h>
> +
> +static long do_fincore(struct address_space *mapping, pgoff_t pgstart,
> + unsigned long nr_pages, unsigned char *vec)
> +{
> + pgoff_t pgend = pgstart + nr_pages;
> + struct radix_tree_iter iter;
> + void **slot;
> + long nr = 0;
> +
> + rcu_read_lock();
> +restart:
> + radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, pgstart) {
> + unsigned char present;
> + struct page *page;
> +
> + /* Handle holes */
> + if (iter.index != pgstart + nr) {
> + if (iter.index < pgend)
> + nr_pages = iter.index - pgstart;
> + break;
> + }
> +repeat:
> + page = radix_tree_deref_slot(slot);
> + if (unlikely(!page))
> + continue;
> +
> + if (radix_tree_exception(page)) {
> + if (radix_tree_deref_retry(page)) {
> + /*
> + * Transient condition which can only trigger
> + * when entry at index 0 moves out of or back
> + * to root: none yet gotten, safe to restart.
> + */
> + WARN_ON(iter.index);
> + goto restart;
> + }
> + present = 0;
> + } else {
> + if (!page_cache_get_speculative(page))
> + goto repeat;
> +
> + /* Has the page moved? */
> + if (unlikely(page != *slot)) {
> + page_cache_release(page);
> + goto repeat;
> + }
> +
> + present = PageUptodate(page);
> + page_cache_release(page);
> + }
> + vec[nr] = present;
> +
> + if (++nr == nr_pages)
> + break;
> + }
> + rcu_read_unlock();
> +
> + if (nr < nr_pages)
> + memset(vec + nr, 0, nr_pages - nr);
> +
> + return nr_pages;
> +}
> +
> +/*
> + * The fincore(2) system call.
> + *
> + * fincore() returns the memory residency status of the given file's
> + * pages, in the range [start, start + len].
> + * The status is returned in a vector of bytes. The least significant
> + * bit of each byte is 1 if the referenced page is in memory, otherwise
> + * it is zero.
> + *
> + * Because the status of a page can change after fincore() checks it
> + * but before it returns to the application, the returned vector may
> + * contain stale information.
> + *
> + * return values:
> + * zero - success
> + * -EBADF - fd isn't a valid open file descriptor
> + * -EFAULT - vec points to an illegal address
> + * -EINVAL - start is not a multiple of PAGE_CACHE_SIZE
> + */
> +SYSCALL_DEFINE4(fincore, unsigned int, fd, loff_t, start, loff_t, len,
> + unsigned char __user *, vec)
> +{
> + unsigned long nr_pages;
> + pgoff_t pgstart;
> + struct fd f;
> + long ret;
> +
> + if (start & ~PAGE_CACHE_MASK)
> + return -EINVAL;
> +
> + f = fdget(fd);
> + if (!f.file)
> + return -EBADF;
> +
> + pgstart = start >> PAGE_CACHE_SHIFT;
> + nr_pages = DIV_ROUND_UP(len, PAGE_CACHE_SIZE);
> +
> + while (nr_pages) {
> + unsigned char tmp[64];
> +
> + ret = do_fincore(f.file->f_mapping, pgstart,
> + min(nr_pages, sizeof(tmp)), tmp);
> + if (ret <= 0)
> + break;
> +
> + if (copy_to_user(vec, tmp, ret)) {
> + ret = -EFAULT;
> + break;
> + }
> +
> + nr_pages -= ret;
> + pgstart += ret;
> + vec += ret;
> + ret = 0;
> + }
> +
> + fdput(f);
> +
> + return ret;
> +}
--
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