linux-ext4 - Re: [PATCH v2 2/3] mm, x86: Add support for PUD-sized transparent hugepages

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Date:   Thu, 26 Jan 2017 14:38:54 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Dave Jiang <dave.jiang@...el.com>
Cc:     dave.hansen@...ux.intel.com, mawilcox@...rosoft.com,
        linux-nvdimm@...1.01.org, linux-xfs@...r.kernel.org,
        linux-mm@...ck.org, vbabka@...e.cz, jack@...e.com,
        dan.j.williams@...el.com, linux-ext4@...r.kernel.org,
        ross.zwisler@...ux.intel.com, kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH v2 2/3] mm, x86: Add support for PUD-sized transparent
 hugepages

On Thu, 26 Jan 2017 10:09:53 -0700 Dave Jiang <dave.jiang@...el.com> wrote:

> The current transparent hugepage code only supports PMDs.  This patch
> adds support for transparent use of PUDs with DAX.  It does not include
> support for anonymous pages. x86 support code also added.
> 
> Most of this patch simply parallels the work that was done for huge PMDs.
> The only major difference is how the new ->pud_entry method in mm_walk
> works.  The ->pmd_entry method replaces the ->pte_entry method, whereas
> the ->pud_entry method works along with either ->pmd_entry or ->pte_entry.
> The pagewalk code takes care of locking the PUD before calling ->pud_walk,
> so handlers do not need to worry whether the PUD is stable.

The patch adds a lot of new BUG()s and BG_ON()s.  We'll get in trouble
if any of those triggers.  Please recheck everything and decide if we
really really need them.  It's far better to drop a WARN and to back
out and recover in some fashion.