Re: [PATCH] zram: Using GFP_ATOMIC instead of GFP_KERNEL to allocate bitmap memory in backing_dev_store

[Dongyun Liu]

On 2023/11/30 23:37, Jens Axboe wrote:
> On 11/30/23 8:20 AM, Dongyun Liu wrote:
> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > index d77d3664ca08..ee6c22c50e09 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -514,7 +514,7 @@ static ssize_t backing_dev_store(struct device *dev,
> >   
> >   	nr_pages = i_size_read(inode) >> PAGE_SHIFT;
> >   	bitmap_sz = BITS_TO_LONGS(nr_pages) * sizeof(long);
> > -	bitmap = kvzalloc(bitmap_sz, GFP_KERNEL);
> > +	bitmap = kmalloc(bitmap_sz, GFP_ATOMIC);
> >   	if (!bitmap) {
> >   		err = -ENOMEM;
> >   		goto out;
>
> Outside of this moving from a zeroed alloc to one that does not, the
> change looks woefully incomplete. Why does this allocation need to be
> GFP_ATOMIC, and:

By using GFP_ATOMIC, it indicates that the caller cannot reclaim or 
sleep, although we can prevent the risk of  deadlock when acquiring the 
zram->lock again in zram_bvec_write.

> 1) file_name = kmalloc(PATH_MAX, GFP_KERNEL); does not

There is no zram->init_lock held here, so there is no need to use
GFP_ATOMIC.

> 2) filp_open() -> getname_kernel() -> __getname() does not
> 3) filp_open() -> getname_kernel() does not
> 4) bdev_open_by_dev() does not

Missing the use of GFP_ATOMIC.

> IOW, you have a slew of GFP_KERNEL allocations in there, and you
> probably just patched the largest one. But the core issue remains.
>
> The whole handling of backing_dev_store() looks pretty broken.

Indeed, this patch only solves the biggest problem and does not
fundamentally solve it, because there are many processes for holding
zram->init_lock before allocation memory in backing_dev_store that need
to be fully modified, and I did not consider it thoroughly. Obviously,
a larger and better patch is needed to eliminate this risk, but it is 
currently not necessary.

Thank you for your kind and patient.