Re: [PATCH v3] mm/vmalloc: fix spinning drain_vmap_work after reading from /proc/vmcore

[Baoquan He <bhe@redhat.com>]

On 04/13/22 at 09:28am, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> Commit 3ee48b6af49c ("mm, x86: Saving vmcore with non-lazy freeing of
> vmas") introduced set_iounmap_nonlazy(), which sets vmap_lazy_nr to
> lazy_max_pages() + 1, ensuring that any future vunmaps() immediately
> purge the vmap areas instead of doing it lazily.
> 
> Commit 690467c81b1a ("mm/vmalloc: Move draining areas out of caller
> context") moved the purging from the vunmap() caller to a worker thread.
> Unfortunately, set_iounmap_nonlazy() can cause the worker thread to spin
> (possibly forever). For example, consider the following scenario:
> 
> 1. Thread reads from /proc/vmcore. This eventually calls
>    __copy_oldmem_page() -> set_iounmap_nonlazy(), which sets
>    vmap_lazy_nr to lazy_max_pages() + 1.
> 2. Then it calls free_vmap_area_noflush() (via iounmap()), which adds 2
>    pages (one page plus the guard page) to the purge list and
>    vmap_lazy_nr. vmap_lazy_nr is now lazy_max_pages() + 3, so the
>    drain_vmap_work is scheduled.
> 3. Thread returns from the kernel and is scheduled out.
> 4. Worker thread is scheduled in and calls drain_vmap_area_work(). It
>    frees the 2 pages on the purge list. vmap_lazy_nr is now
>    lazy_max_pages() + 1.
> 5. This is still over the threshold, so it tries to purge areas again,
>    but doesn't find anything.
> 6. Repeat 5.
> 
> If the system is running with only one CPU (which is typicial for kdump)
> and preemption is disabled, then this will never make forward progress:
> there aren't any more pages to purge, so it hangs. If there is more than
> one CPU or preemption is enabled, then the worker thread will spin
> forever in the background. (Note that if there were already pages to be
> purged at the time that set_iounmap_nonlazy() was called, this bug is
> avoided.)
> 
> This can be reproduced with anything that reads from /proc/vmcore
> multiple times. E.g., vmcore-dmesg /proc/vmcore.
> 
> It turns out that improvements to vmap() over the years have obsoleted
> the need for this "optimization". I benchmarked
> `dd if=/proc/vmcore of=/dev/null` with 4k and 1M read sizes on a system
> with a 32GB vmcore. The test was run on 5.17, 5.18-rc1 with a fix that
> avoided the hang, and 5.18-rc1 with set_iounmap_nonlazy() removed
> entirely:
> 
>   |5.17  |5.18+fix|5.18+removal
> 4k|40.86s|  40.09s|      26.73s
> 1M|24.47s|  23.98s|      21.84s
> 
> The removal was the fastest (by a wide margin with 4k reads). This patch
> removes set_iounmap_nonlazy().
> 
> Fixes: 690467c81b1a ("mm/vmalloc: Move draining areas out of caller context")
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Acked-by: Chris Down <chris@chrisdown.name>
> Signed-off-by: Omar Sandoval <osandov@fb.com>

This is a great fix and with detailed explanation, thanks.

Acked-by: Baoquan He <bhe@redhat.com>

> ---
> Changes from v2 -> v3:
> 
> - Add Fixes and Reviewed-by tags (no code changes)
> 
> Changes from v1 -> v2:
> 
> - Remove set_iounmap_nonlazy() entirely instead of fixing it.
> 
>  arch/x86/include/asm/io.h       |  2 --
>  arch/x86/kernel/crash_dump_64.c |  1 -
>  mm/vmalloc.c                    | 11 -----------
>  3 files changed, 14 deletions(-)
> 
> diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
> index f6d91ecb8026..e9736af126b2 100644
> --- a/arch/x86/include/asm/io.h
> +++ b/arch/x86/include/asm/io.h
> @@ -210,8 +210,6 @@ void __iomem *ioremap(resource_size_t offset, unsigned long size);
>  extern void iounmap(volatile void __iomem *addr);
>  #define iounmap iounmap
>  
> -extern void set_iounmap_nonlazy(void);
> -
>  #ifdef __KERNEL__
>  
>  void memcpy_fromio(void *, const volatile void __iomem *, size_t);
> diff --git a/arch/x86/kernel/crash_dump_64.c b/arch/x86/kernel/crash_dump_64.c
> index a7f617a3981d..97529552dd24 100644
> --- a/arch/x86/kernel/crash_dump_64.c
> +++ b/arch/x86/kernel/crash_dump_64.c
> @@ -37,7 +37,6 @@ static ssize_t __copy_oldmem_page(unsigned long pfn, char *buf, size_t csize,
>  	} else
>  		memcpy(buf, vaddr + offset, csize);
>  
> -	set_iounmap_nonlazy();
>  	iounmap((void __iomem *)vaddr);
>  	return csize;
>  }
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index e163372d3967..0b17498a34f1 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1671,17 +1671,6 @@ static DEFINE_MUTEX(vmap_purge_lock);
>  /* for per-CPU blocks */
>  static void purge_fragmented_blocks_allcpus(void);
>  
> -#ifdef CONFIG_X86_64
> -/*
> - * called before a call to iounmap() if the caller wants vm_area_struct's
> - * immediately freed.
> - */
> -void set_iounmap_nonlazy(void)
> -{
> -	atomic_long_set(&vmap_lazy_nr, lazy_max_pages()+1);
> -}
> -#endif /* CONFIG_X86_64 */
> -
>  /*
>   * Purges all lazily-freed vmap areas.
>   */
> -- 
> 2.35.2
> 
> 
>