linux-kernel - Re: [PATCH 0/9] Mitigate a vmap lock contention

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Date:   Thu, 25 May 2023 07:56:56 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, Baoquan He <bhe@...hat.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH 0/9] Mitigate a vmap lock contention

On Wed, May 24, 2023 at 11:50:12AM +0200, Uladzislau Rezki wrote:
> On Wed, May 24, 2023 at 03:04:28AM +0900, Hyeonggon Yoo wrote:
> > On Tue, May 23, 2023 at 05:12:30PM +0200, Uladzislau Rezki wrote:
> > And I would like to ask some side questions:
> > 
> > 1. Is vm_[un]map_ram() API still worth with this patchset?
> > 
> It is up to community to decide. As i see XFS needs it also. Maybe in
> the future it can be removed(who knows). If the vmalloc code itself can
> deliver such performance as vm_map* APIs.

vm_map* APIs cannot be replaced with vmalloc, they cover a very
different use case.  i.e.  vmalloc allocates mapped memory,
vm_map_ram() maps allocated memory....

> vm_map_ram() and friends interface was added because of vmalloc drawbacks.

No. vm_map*() were scalability improvements added in 2009 to replace
on vmap() and vunmap() to avoid global lock contention in the vmap
allocator that XFS had been working around for years with it's own
internal vmap cache....

commit 95f8e302c04c0b0c6de35ab399a5551605eeb006
Author: Nicholas Piggin <npiggin@...il.com>
Date:   Tue Jan 6 14:43:09 2009 +1100

    [XFS] use scalable vmap API
    
    Implement XFS's large buffer support with the new vmap APIs. See the vmap
    rewrite (db64fe02) for some numbers. The biggest improvement that comes from
    using the new APIs is avoiding the global KVA allocation lock on every call.
    
    Signed-off-by: Nick Piggin <npiggin@...e.de>
    Reviewed-by: Christoph Hellwig <hch@...radead.org>
    Signed-off-by: Lachlan McIlroy <lachlan@....com>

vmap/vunmap() themselves were introduce in 2.5.32 (2002) and before
that XFS was using remap_page_array() and vfree() in exactly the
same way it uses vm_map_ram() and vm_unmap_ram() today....

XFS has a long, long history of causing virtual memory allocator
scalability and contention problems. As you can see, this isn't our
first rodeo...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com