Re: [PATCH 2/2] Release mmap_sem when page fault blocks on disk transfer.

[Rik van Riel <riel@redhat.com>]

On 10/01/2010 01:04 AM, Michel Lespinasse wrote:
> This change reduces mmap_sem hold times that are caused by waiting for
> disk transfers when accessing file mapped VMAs. It introduces the
> VM_FAULT_RELEASED flag, which indicates that the call site holds mmap_lock
> and wishes for it to be released if blocking on a pending disk transfer.
> In that case, filemap_fault() returns the VM_FAULT_RELEASED status bit
> and do_page_fault() will then re-acquire mmap_sem and retry the page fault.
> It is expected that the retry will hit the same page which will now be cached,
> and thus it will complete with a low mmap_sem hold time.

The concept makes sense.  A nitpick, though...

> +	if (release_flag) {	/* Did not go through a retry */
> +		if (fault&  VM_FAULT_MAJOR) {
> +			tsk->maj_flt++;
> +			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, 0,
> +				      regs, address);
> +		} else {
> +			tsk->min_flt++;
> +			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, 0,
> +				      regs, address);
> +		}
> +		if (fault&  VM_FAULT_RELEASED) {
> +			/*
> +			 * handle_mm_fault() found that the desired page was
> +			 * locked. We asked for it to release mmap_sem in that
> +			 * case, so as to avoid holding it for too long.
> +			 * Retry starting at the mmap_sem acquire, this time
> +			 * without FAULT_FLAG_RETRY so that we avoid any
> +			 * risk of starvation.
> +			 */
> +			release_flag = 0;
> +			goto retry;
> +		}

Do we really want to count a minor page fault when we
got VM_FAULT_RELEASED?

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