linux-kernel - Re: [PATCH v1] mm:free unused pages in kmalloc

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Message-ID: <20200629145259.GD25523@casper.infradead.org>
Date:   Mon, 29 Jun 2020 15:52:59 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Long Li <lonuxli.64@...il.com>, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm:free unused pages in kmalloc_order

On Mon, Jun 29, 2020 at 02:48:06PM +0000, Christopher Lameter wrote:
> On Sat, 27 Jun 2020, Long Li wrote:
> > Environment using the slub allocator, 1G memory in my ARM32.
> > kmalloc(1024, GFP_HIGHUSER) can allocate memory normally,
> > kmalloc(64*1024, GFP_HIGHUSER) will cause a memory leak, because
> > alloc_pages returns highmem physical pages, but it cannot be directly
> > converted into a virtual address and return NULL, the pages has not
> > been released. Usually driver developers will not use the
> > GFP_HIGHUSER flag to allocate memory in kmalloc, but I think this
> > memory leak is not perfect, it is best to be fixed. This is the
> > first time I have posted a patch, there may be something wrong.
> 
> Highmem is not supported by the slab allocators. Please ensure that there
> is a warning generated if someone attempts to do such an allocation. We
> used to check for that.

Sounds like we need a test somewhere that checks this behaviour.

> In order to make such allocations possible one would have to create yet
> another kmalloc array for high memory.

Not for this case because it goes straight to kmalloc_order().  What does
make this particular case impossible is that we can't kmap() a compound
page.  We could vmap it, but why are we bothering?