Re: [RFC PATCH 1/8] fat: support FIEMAP

[Matheus Alves de Almeida]

On Thu, Nov 12, 2026 at 02:03:45PM +0900, OGAWA Hirofumi wrote:
>Matheus Alves de Almeida <matheus.aalmeida@xxxxxxxxxxxx> writes:
>
>
>[...]
>
>>>>  	if (fat_cache_lookup(inode, cluster, &cid, fclus, dclus) < 0) {
>>>> -		/*
>>>> -		 * dummy, always not contiguous
>>>> -		 * This is reinitialized by cache_init(), later.
>>>> -		 */
>>>> -		cache_init(&cid, -1, -1);
>>>> +		/* Cache the contiguous prefix from the known first cluster. */
>>>> +		cache_init(&cid, 0, *dclus);
>>>
>>>Doesn't this add a needless cache? 0 is cached in the inode itself,
>>>isn't it? (and can be waste small fatent cache slots?)
>>>
>>
>> This is mostly for caching the number of contiguous clusters starting
>> from 0. i_start only really tells us the first cluster of the file. I'm
>> not exactly sure of the best way to approach this, though. I guess we
>> can skip caching when its a one cluster range in this case?
>>
>> What do you think?
>
>Ah, sure. I think because it is fat_cache_lookup() and stuff are
>implemented for nr_cont == 1. So I think better way would be supporting
>the range lookup in whole cache stuff. (however, it can be relatively
>complex)
>
>Thanks.
>

It seems like exfat had to solve a similar problem a while back:
https://lore.kernel.org/linux-fsdevel/20260114121250.615064-1-chizhiling@xxxxxxx/

Maybe I can work on porting some of that to fat. Would you rather it be
in a separate series or just add it to the iomap series?

Thanks,
Matheus