Andreas Dilger <adilger@xxxxxxxxx> 于2023年8月4日周五 06:34写道：
>
> On Aug 2, 2023, at 9:09 PM, Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
> >
> > On Thu, Aug 03, 2023 at 09:52:53AM +0800, Stephen Zhang wrote:
> >> Andreas Dilger <adilger@xxxxxxxxx> 于2023年8月2日周三 14:07写道：
> >>>
> >>> Not all of these cases are actual bugs. The ext4_rec_len_from_disk()
> >>> function is only different for rec_len >= 2^16, so if it is comparing
> >>> rec_len against "12" or "sizeof(struct ...)" then the inequality will
> >>> be correct regardless of how it is decoded.
> >>>
> >>> That said, it makes sense to use ext4_rec_len_from_disk() to access
> >>> rec_len consistently throughout the code, since that avoids potential
> >>> bugs in the future. We know the code will eventually will be copied
> >>> some place where rec_len >= 2^16 is actually important, and we may as
> >>> well avoid that bug before it happens.
> >>>
> >>>
> >>> One thing this discussion *does* expose is that ext4_rec_len_from_disk()
> >>> is hard-coded at compile time to differentiate between PAGE_SIZE > 64k
> >>> and PAGE_SIZE = 4K, because it was never possible to have blocksize >
> >>> PAGE_SIZE, so only ARM/PPC ever had filesystems with blocksize=64KiB
> >>> (and the Fujitsu Fugaku SPARC system with blocksize=256KiB).
> >>>
> >>> However, with the recent advent of the VM and IO layers allowing
> >>> blocksize > PAGE_SIZE this function will need to be changed to allow
> >>> the same on x86 PAGE_SIZE=4KiB systems. Instead of checking
> >>>
> >>> #if PAGE_SIZE >= 65536
> >>>
> >>> it should handle this based on the filesystem blocksize at runtime:
> >>>
> >>> static inline
> >>> unsigned int ext4_rec_len_from_disk(__le16 dlen, unsigned blocksize)
> >>> {
> >>> unsigned len = le16_to_cpu(dlen);
> >>>
> >>> if (blocksize < 65536)
> >>> return len;
> >>>
> >>> if (len == EXT4_MAX_REC_LEN || len == 0)
> >>> return blocksize;
> >>>
> >>> return (len & 65532) | ((len & 3) << 16);
> >>> }
> >>>
> >>> Strictly speaking, ((len & 65532) | ((len & 3) << 16) should equal "len"
> >>> for any filesystem with blocksize < 65536, but IMHO it is more clear if
> >>> the code is written this way.
> >>>
> >>> Similarly, the encoding needs to be changed to handle large records at
> >>> runtime for when we eventually allow ext4 with blocksize > PAGE_SIZE.
> >>>
> >>> static inline __le16 ext4_rec_len_to_disk(unsigned len, unsigned blocksize)
> >>> {
> >>> BUG_ON(len > blocksize);
> >>> BUG_ON(blocksize > (1 << 18));
> >>> BUG_ON(len & 3);
> >>>
> >>> if (len < 65536) /* always true for blocksize < 65536 */
> >>> return cpu_to_le16(len);
> >>>
> >>> if (len == blocksize) {
> >>> if (blocksize == 65536)
> >>> return cpu_to_le16(EXT4_MAX_REC_LEN);
> >>>
> >>> return cpu_to_le16(0);
> >>> }
> >>>
> >>> return cpu_to_le16((len & 65532) | ((len >> 16) & 3));
> >>> }
> >>>
> >>
> >> Hmm, at least it sounds reasonable to me based on my limited
> >> knowledge. However, I am not sure whether you want me to incorporate
> >> these changes into this particular commit or another patch within this
> >> submission.
> >>
> >> By default, I will simply leave it for further discussion. Please let
> >> me know if you have any ideas.
> >
> > ext4 doesn't support blocksize > PAGE_SIZE yet. Don't worry about this
> > for now.
>
> I agree it doesn't need to be merged into the current patch.
>
> It's something that could be fixed in a follow-on patch, to have one less
> bug to fix in the future when ext4 *does* support blocksize > PAGE_SIZE,
> which isn't so far away anymore.
>
Okay, I will attempt to submit another follow-on patch based on this discussion
after this one.
Cheers,
Shida
> Cheers, Andreas
>
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