linux-kernel - Re: [PATCH] filesystem-dax: Disable PMD support

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Message-ID: <20190703154700.GI1729@bombadil.infradead.org>
Date:   Wed, 3 Jul 2019 08:47:00 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Seema Pandit <seema.pandit@...el.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Robert Barror <robert.barror@...el.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] filesystem-dax: Disable PMD support

On Mon, Jul 01, 2019 at 02:11:19PM +0200, Jan Kara wrote:
> BTW, looking into the xarray code, I think I found another difference
> between the old radix tree code and the new xarray code that could cause
> issues. In the old radix tree code if we tried to insert PMD entry but
> there was some PTE entry in the covered range, we'd get EEXIST error back
> and the DAX fault code relies on this. I don't see how similar behavior is
> achieved by xas_store()...

Are you referring to this?

-               entry = dax_make_locked(0, size_flag | DAX_EMPTY);
-
-               err = __radix_tree_insert(&mapping->i_pages, index,
-                               dax_entry_order(entry), entry);
-               radix_tree_preload_end();
-               if (err) {
-                       xa_unlock_irq(&mapping->i_pages);
-                       /*
-                        * Our insertion of a DAX entry failed, most likely
-                        * because we were inserting a PMD entry and it
-                        * collided with a PTE sized entry at a different
-                        * index in the PMD range.  We haven't inserted
-                        * anything into the radix tree and have no waiters to
-                        * wake.
-                        */
-                       return ERR_PTR(err);
-               }

If so, that can't happen any more because we no longer drop the i_pages
lock while the entry is NULL, so the entry is always locked while the
i_pages lock is dropped.