From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6A2831991B8 for ; Mon, 18 Nov 2024 20:00:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731960058; cv=none; b=OeFuibqe1MQWKnasvPKIn5NmFRyNnRgbKmZ1nX8NZTHafz22kItsWdtgohZUQPxFcbccVEwmGMnxNjmMvP0ZQdJmrtDbGqdFeYGe8BCsWagllllVbO9fltHVNqZ/SqkJc74OR1uc1srY316DBEM08AZ9IiDbru0M+gMdjxDgMU0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731960058; c=relaxed/simple; bh=6ZfG54qMaBmtojfIQ1yqdSaeMf2o3Vn1qGAASG3i8j8=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=OvsqWgol83PFbDNj+6QuniKN0p0Mxou0VI+zTrlz+VadT8cwTQk5VQTF/Xmdnors53gDjYXFI/VYZY7IQEolJUqxKWZYBHRsOPkYPl1s9/juRlI1Lj8uS4d37tC6XbUYQw4gylkFdjcdyabKdCYBq9be4JOfHHgqBKZVE7nRipc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=C+VEr/Zv; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="C+VEr/Zv" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4BC15C4CECC; Mon, 18 Nov 2024 20:00:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1731960058; bh=6ZfG54qMaBmtojfIQ1yqdSaeMf2o3Vn1qGAASG3i8j8=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=C+VEr/Zvxtje8c/clxxc9iPck++nfEMUh0erB/RtcEIBbt6/A9hwXlSjNc4HLT3IH rDiJCTKh9bj36TzCnD30+WdIWEvBKRyLlv0T+AKYaiV1uEA/Kt/gJBY5+em6SpmQ36 a3XgrOhpNP89XhBfPf4XIDpYQsMGWWxOD1Z29cuISHJS+Mjd1AHV0fClvHIF0bg7Dd f6O9Svkqo0tGTYHSsuj5+5ZPEUNCh1MhezMn0N32QW4J5pFB3jIryi/Z4muAuqVpxz p443itFh9oCZD0AczVOsk4/emYjKnGpj/WSWpKK6MmyCv1Wtgi1otVOqxRqtkchmeY +ZU0tnuXMbO4A== Message-ID: Subject: Re: [RFC PATCH 2/2] libfs: Improve behavior when directory offset values wrap From: Jeff Layton To: cel@kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Hugh Dickens Cc: yukuai3@huawei.com, yangerkun@huaweicloud.com, Chuck Lever Date: Mon, 18 Nov 2024 15:00:56 -0500 In-Reply-To: <20241117213206.1636438-3-cel@kernel.org> References: <20241117213206.1636438-1-cel@kernel.org> <20241117213206.1636438-3-cel@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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>=20 > The fix in commit 64a7ce76fb90 ("libfs: fix infinite directory reads > for offset dir") introduced a fence in offset_iterate_dir() to stop > the loop from returning child entries created after the directory > was opened. This comparison relies on the strong ordering of > DIR_OFFSET_MIN <=3D largest child offset <=3D next_offset to terminate > the directory iteration. >=20 > However, because simple_offset_add() uses mtree_alloc_cyclic() to > select each next new directory offset, ctx->next_offset is not > always the highest unused offset. Once mtree_alloc_cyclic() allows > a new offset value to wrap, ctx->next_offset will be set to a value > less than the actual largest child offset. >=20 > The result is that readdir(3) no longer shows any entries in the > directory because their offsets are above ctx->next_offset, which is > now a small value. This situation is persistent, and the directory > cannot be removed unless all current children are already known and > can be explicitly removed by name first. >=20 > In the current Maple tree implementation, there is no practical way > that 63-bit offset values can ever wrap, so this issue is cleverly > avoided. But the ordering dependency is not documented via comments > or code, making the mechanism somewhat brittle. And it makes the > continued use of mtree_alloc_cyclic() somewhat confusing. >=20 > Further, if commit 64a7ce76fb90 ("libfs: fix infinite directory > reads for offset dir") were to be backported to a kernel that still > uses xarray to manage simple directory offsets, the directory offset > value range is limited to 32-bits, which is small enough to allow a > wrap after a few weeks of constant creation of entries in one > directory. >=20 > Therefore, replace the use of ctx->next_offset for fencing new > children from appearing in readdir results. >=20 > A jiffies timestamp marks the end of each opendir epoch. Entries > created after this timestamp will not be visible to the file > descriptor. I chose jiffies so that the dentry->d_time field can be > re-used for storing the entry creation time. >=20 > The new mechanism has its own corner cases. For instance, I think > if jiffies wraps twice while a directory is open, some children > might become invisible. On 32-bit systems, the jiffies value wraps > every 49 days. Double-wrapping is not a risk on systems with 64-bit > jiffies. Unlike with the next_offset-based mechanism, re-opening the > directory will make invisible children re-appear. >=20 > Reported-by: Yu Kuai > Closes: https://lore.kernel.org/stable/20241111005242.34654-1-cel@kernel.= org/T/#m1c448e5bd4aae3632a09468affcfe1d1594c6a59 > Fixes: 64a7ce76fb90 ("libfs: fix infinite directory reads for offset dir"= ) > Signed-off-by: Chuck Lever > --- > fs/libfs.c | 36 +++++++++++++++++------------------- > 1 file changed, 17 insertions(+), 19 deletions(-) >=20 > diff --git a/fs/libfs.c b/fs/libfs.c > index bf67954b525b..862a603fd454 100644 > --- a/fs/libfs.c > +++ b/fs/libfs.c > @@ -294,6 +294,7 @@ int simple_offset_add(struct offset_ctx *octx, struct= dentry *dentry) > return ret; > =20 > offset_set(dentry, offset); > + WRITE_ONCE(dentry->d_time, jiffies); > return 0; > } > =20 > @@ -454,9 +455,7 @@ void simple_offset_destroy(struct offset_ctx *octx) > =20 > static int offset_dir_open(struct inode *inode, struct file *file) > { > - struct offset_ctx *ctx =3D inode->i_op->get_offset_ctx(inode); > - > - file->private_data =3D (void *)ctx->next_offset; > + file->private_data =3D (void *)jiffies; > return 0; > } > =20 > @@ -473,9 +472,6 @@ static int offset_dir_open(struct inode *inode, struc= t file *file) > */ > static loff_t offset_dir_llseek(struct file *file, loff_t offset, int wh= ence) > { > - struct inode *inode =3D file->f_inode; > - struct offset_ctx *ctx =3D inode->i_op->get_offset_ctx(inode); > - > switch (whence) { > case SEEK_CUR: > offset +=3D file->f_pos; > @@ -490,7 +486,8 @@ static loff_t offset_dir_llseek(struct file *file, lo= ff_t offset, int whence) > =20 > /* In this case, ->private_data is protected by f_pos_lock */ > if (!offset) > - file->private_data =3D (void *)ctx->next_offset; > + /* Make newer child entries visible */ > + file->private_data =3D (void *)jiffies; > return vfs_setpos(file, offset, LONG_MAX); > } > =20 > @@ -521,7 +518,8 @@ static bool offset_dir_emit(struct dir_context *ctx, = struct dentry *dentry) > inode->i_ino, fs_umode_to_dtype(inode->i_mode)); > } > =20 > -static void offset_iterate_dir(struct inode *inode, struct dir_context *= ctx, long last_index) > +static void offset_iterate_dir(struct inode *inode, struct dir_context *= ctx, > + unsigned long fence) > { > struct offset_ctx *octx =3D inode->i_op->get_offset_ctx(inode); > struct dentry *dentry; > @@ -531,14 +529,15 @@ static void offset_iterate_dir(struct inode *inode,= struct dir_context *ctx, lon > if (!dentry) > return; > =20 > - if (dentry2offset(dentry) >=3D last_index) { > - dput(dentry); > - return; > - } > - > - if (!offset_dir_emit(ctx, dentry)) { > - dput(dentry); > - return; > + /* > + * Output only child entries created during or before > + * the current opendir epoch. > + */ > + if (time_before_eq(dentry->d_time, fence)) { > + if (!offset_dir_emit(ctx, dentry)) { > + dput(dentry); > + return; > + } > } > =20 > ctx->pos =3D dentry2offset(dentry) + 1; > @@ -569,15 +568,14 @@ static void offset_iterate_dir(struct inode *inode,= struct dir_context *ctx, lon > */ > static int offset_readdir(struct file *file, struct dir_context *ctx) > { > + unsigned long fence =3D (unsigned long)file->private_data; > struct dentry *dir =3D file->f_path.dentry; > - long last_index =3D (long)file->private_data; > =20 > lockdep_assert_held(&d_inode(dir)->i_rwsem); > =20 > if (!dir_emit_dots(file, ctx)) > return 0; > - > - offset_iterate_dir(d_inode(dir), ctx, last_index); > + offset_iterate_dir(d_inode(dir), ctx, fence); > return 0; > } > =20 Using timestamps instead of directory ordering does seem less brittle, and the choice to use jiffies makes sense given that d_time is also an unsigned long. Reviewed-by: Jeff Layton