Re: [PATCH] mm: don't cap request size based on read-ahead setting

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On 11/16/2016 08:12 AM, Jens Axboe wrote:
On 11/16/2016 12:17 AM, Hillf Danton wrote:
On Wednesday, November 16, 2016 12:31 PM Jens Axboe wrote:
@@ -369,10 +369,25 @@ ondemand_readahead(struct address_space *mapping,
             bool hit_readahead_marker, pgoff_t offset,
             unsigned long req_size)
  {
-    unsigned long max = ra->ra_pages;
+    unsigned long io_pages, max_pages;
      pgoff_t prev_offset;

      /*
+     * If bdi->io_pages is set, that indicates the (soft) max IO size
+     * per command for that device. If we have that available, use
+     * that as the max suitable read-ahead size for this IO. Instead of
+     * capping read-ahead at ra_pages if req_size is larger, we can go
+     * up to io_pages. If io_pages isn't set, fall back to using
+     * ra_pages as a safe max.
+     */
+    io_pages = inode_to_bdi(mapping->host)->io_pages;
+    if (io_pages) {
+        max_pages = max_t(unsigned long, ra->ra_pages, req_size);
+        io_pages = min(io_pages, max_pages);

Doubt if you mean
        max_pages = min(io_pages, max_pages);

No, that is what I mean. We want the maximum of the RA setting and the
user IO size, but the minimum of that and the device max command size.

Johannes pointed out that I'm an idiot - a last minute edit introduced
this typo, and I was too blind to spot it when you sent that email this
morning. So yes, it should of course be:

	max_pages = min(io_pages, max_pages);

like the first version I posted. I'll post a v3...

--
Jens Axboe

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